diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst
index 9ec1a9d0e03e..aa6f7d84e583 100644
--- a/docs/system/deprecated.rst
+++ b/docs/system/deprecated.rst
@@ -315,26 +315,6 @@ this CPU is also deprecated.
 Related binaries
 ----------------

-qemu-img backing file without format (since 5.1)
-''''''''''''''''''''''''''''''''''''''''''''''''
-
-The use of ``qemu-img create``, ``qemu-img rebase``, or ``qemu-img
-convert`` to create or modify an image that depends on a backing file
-now recommends that an explicit backing format be provided.  This is
-for safety: if QEMU probes a different format than what you thought,
-the data presented to the guest will be corrupt; similarly, presenting
-a raw image to a guest allows a potential security exploit if a future
-probe sees a non-raw image based on guest writes.
-
-To avoid the warning message, or even future refusal to create an
-unsafe image, you must pass ``-o backing_fmt=`` (or the shorthand
-``-F`` during create) to specify the intended backing format.  You may
-use ``qemu-img rebase -u`` to retroactively add a backing format to an
-existing image.  However, be aware that there are already potential
-security risks to blindly using ``qemu-img info`` to probe the format
-of an untrusted backing image, when deciding what format to add into
-an existing image.
-
 Backwards compatibility
 -----------------------

diff --git a/docs/system/removed-features.rst b/docs/system/removed-features.rst
index 28b5df757d35..1928d8a483c0 100644
--- a/docs/system/removed-features.rst
+++ b/docs/system/removed-features.rst
@@ -466,6 +466,25 @@ backing chain should be performed with ``qemu-img rebase -u`` either
 before or after the remaining changes being performed by amend, as
 appropriate.

+qemu-img backing file without format (removed in 6.1)
+'''''''''''''''''''''''''''''''''''''''''''''''''''''
+
+The use of ``qemu-img create``, ``qemu-img rebase``, or ``qemu-img
+convert`` to create or modify an image that depends on a backing file
+now requires that an explicit backing format be provided.  This is
+for safety: if QEMU probes a different format than what you thought,
+the data presented to the guest will be corrupt; similarly, presenting
+a raw image to a guest allows a potential security exploit if a future
+probe sees a non-raw image based on guest writes.
+
+To avoid creating unsafe backing chains, you must pass ``-o
+backing_fmt=`` (or the shorthand ``-F`` during create) to specify the
+intended backing format.  You may use ``qemu-img rebase -u`` to
+retroactively add a backing format to an existing image.  However, be
+aware that there are already potential security risks to blindly using
+``qemu-img info`` to probe the format of an untrusted backing image,
+when deciding what format to add into an existing image.
+
 Block devices
 -------------

diff --git a/block.c b/block.c
index 874c22c43e3d..931e37a8499b 100644
--- a/block.c
+++ b/block.c
@@ -5033,7 +5033,7 @@ int coroutine_fn bdrv_co_check(BlockDriverState *bs,
  * -ENOTSUP - format driver doesn't support changing the backing file
  */
 int bdrv_change_backing_file(BlockDriverState *bs, const char *backing_file,
-                             const char *backing_fmt, bool warn)
+                             const char *backing_fmt, bool require)
 {
     BlockDriver *drv = bs->drv;
     int ret;
@@ -5047,10 +5047,8 @@ int bdrv_change_backing_file(BlockDriverState *bs, const char *backing_file,
         return -EINVAL;
     }

-    if (warn && backing_file && !backing_fmt) {
-        warn_report("Deprecated use of backing file without explicit "
-                    "backing format, use of this image requires "
-                    "potentially unsafe format probing");
+    if (require && backing_file && !backing_fmt) {
+        return -EINVAL;
     }

     if (drv->bdrv_change_backing_file != NULL) {
@@ -6556,24 +6554,11 @@ void bdrv_img_create(const char *filename, const char *fmt,
             goto out;
         } else {
             if (!backing_fmt) {
-                warn_report("Deprecated use of backing file without explicit "
-                            "backing format (detected format of %s)",
-                            bs->drv->format_name);
-                if (bs->drv != &bdrv_raw) {
-                    /*
-                     * A probe of raw deserves the most attention:
-                     * leaving the backing format out of the image
-                     * will ensure bs->probed is set (ensuring we
-                     * don't accidentally commit into the backing
-                     * file), and allow more spots to warn the users
-                     * to fix their toolchain when opening this image
-                     * later.  For other images, we can safely record
-                     * the format that we probed.
-                     */
-                    backing_fmt = bs->drv->format_name;
-                    qemu_opt_set(opts, BLOCK_OPT_BACKING_FMT, backing_fmt,
-                                 NULL);
-                }
+                error_setg(&local_err,
+                           "Backing file specified without backing format");
+                error_append_hint(&local_err, "Detected format of %s.",
+                                  bs->drv->format_name);
+                goto out;
             }
             if (size == -1) {
                 /* Opened BS, have no size */
@@ -6590,9 +6575,9 @@ void bdrv_img_create(const char *filename, const char *fmt,
         }
         /* (backing_file && !(flags & BDRV_O_NO_BACKING)) */
     } else if (backing_file && !backing_fmt) {
-        warn_report("Deprecated use of unopened backing file without "
-                    "explicit backing format, use of this image requires "
-                    "potentially unsafe format probing");
+        error_setg(&local_err,
+                   "Backing file specified without backing format");
+        goto out;
     }

     if (size == -1) {
diff --git a/qemu-img.c b/qemu-img.c
index a5993682aad4..3cdfcaa23f6b 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -2505,8 +2505,10 @@ static int img_convert(int argc, char **argv)

     if (out_baseimg_param) {
         if (!qemu_opt_get(opts, BLOCK_OPT_BACKING_FMT)) {
-            warn_report("Deprecated use of backing file without explicit "
-                        "backing format");
+            error_report("Use of backing file requires explicit "
+                         "backing format");
+            ret = -1;
+            goto out;
         }
     }

diff --git a/tests/qemu-iotests/114 b/tests/qemu-iotests/114
index 43cb0bc6c344..3e30b402bc4c 100755
--- a/tests/qemu-iotests/114
+++ b/tests/qemu-iotests/114
@@ -44,16 +44,16 @@ _supported_os Linux
 # qcow2.py does not work too well with external data files
 _unsupported_imgopts data_file

-# Intentionally specify backing file without backing format; demonstrate
-# the difference in warning messages when backing file could be probed.
-# Note that only a non-raw probe result will affect the resulting image.
+# Older qemu-img could set up backing file without backing format; modern
+# qemu can't but we can use qcow2.py to simulate older files.
 truncate -s $((64 * 1024 * 1024)) "$TEST_IMG.orig"
-_make_test_img -b "$TEST_IMG.orig" 64M
+_make_test_img -b "$TEST_IMG.orig" -F raw 64M
+$PYTHON qcow2.py "$TEST_IMG" del-header-ext 0xE2792ACA

 TEST_IMG="$TEST_IMG.base" _make_test_img 64M
 $QEMU_IMG convert -O qcow2 -B "$TEST_IMG.orig" "$TEST_IMG.orig" "$TEST_IMG"
-_make_test_img -b "$TEST_IMG.base" 64M
-_make_test_img -u -b "$TEST_IMG.base" 64M
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT 64M
+_make_test_img -u -b "$TEST_IMG.base" -F $IMGFMT 64M

 # Set an invalid backing file format
 $PYTHON qcow2.py "$TEST_IMG" add-header-ext 0xE2792ACA "foo"
@@ -64,9 +64,9 @@ _img_info
 $QEMU_IO -c "open $TEST_IMG" -c "read 0 4k" 2>&1 | _filter_qemu_io | _filter_testdir
 $QEMU_IO -c "open -o backing.driver=$IMGFMT $TEST_IMG" -c "read 0 4k" | _filter_qemu_io

-# Rebase the image, to show that omitting backing format triggers a warning,
-# but probing now lets us use the backing file.
-$QEMU_IMG rebase -u -b "$TEST_IMG.base" "$TEST_IMG"
+# Rebase the image, to show that backing format is required.
+$QEMU_IMG rebase -u -b "$TEST_IMG.base" "$TEST_IMG" && echo "unexpected pass"
+$QEMU_IMG rebase -u -b "$TEST_IMG.base" -F $IMGFMT "$TEST_IMG"
 $QEMU_IO -c "open $TEST_IMG" -c "read 0 4k" 2>&1 | _filter_qemu_io | _filter_testdir

 # success, all done
diff --git a/tests/qemu-iotests/114.out b/tests/qemu-iotests/114.out
index 0a37d20c82a9..172454401257 100644
--- a/tests/qemu-iotests/114.out
+++ b/tests/qemu-iotests/114.out
@@ -1,12 +1,9 @@
 QA output created by 114
-qemu-img: warning: Deprecated use of backing file without explicit backing format (detected format of raw)
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.orig
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.orig backing_fmt=raw
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
-qemu-img: warning: Deprecated use of backing file without explicit backing format
-qemu-img: warning: Deprecated use of backing file without explicit backing format (detected format of IMGFMT)
+qemu-img: Use of backing file requires explicit backing format
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
-qemu-img: warning: Deprecated use of unopened backing file without explicit backing format, use of this image requires potentially unsafe format probing
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
 image: TEST_DIR/t.IMGFMT
 file format: IMGFMT
 virtual size: 64 MiB (67108864 bytes)
@@ -17,7 +14,7 @@ qemu-io: can't open device TEST_DIR/t.qcow2: Could not open backing file: Unknow
 no file open, try 'help open'
 read 4096/4096 bytes at offset 0
 4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-qemu-img: warning: Deprecated use of backing file without explicit backing format, use of this image requires potentially unsafe format probing
+qemu-img: Could not change the backing file to '/home/eblake/qemu/build/tests/qemu-iotests/scratch/t.qcow2.base': Invalid argument
 read 4096/4096 bytes at offset 0
 4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 *** done
diff --git a/tests/qemu-iotests/301 b/tests/qemu-iotests/301
index 9f943cadbe24..220de1043fa5 100755
--- a/tests/qemu-iotests/301
+++ b/tests/qemu-iotests/301
@@ -3,7 +3,7 @@
 #
 # Test qcow backing file warnings
 #
-# Copyright (C) 2020 Red Hat, Inc.
+# Copyright (C) 2020-2021 Red Hat, Inc.
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -46,7 +46,6 @@ echo "== qcow backed by qcow =="

 TEST_IMG="$TEST_IMG.base" _make_test_img $size
 _make_test_img -b "$TEST_IMG.base" $size
-_img_info
 _make_test_img -b "$TEST_IMG.base" -F $IMGFMT $size
 _img_info

@@ -71,7 +70,6 @@ echo "== qcow backed by raw =="
 rm "$TEST_IMG.base"
 truncate --size=$size "$TEST_IMG.base"
 _make_test_img -b "$TEST_IMG.base" $size
-_img_info
 _make_test_img -b "$TEST_IMG.base" -F raw $size
 _img_info

diff --git a/tests/qemu-iotests/301.out b/tests/qemu-iotests/301.out
index 9004dad6392f..e280658191e1 100644
--- a/tests/qemu-iotests/301.out
+++ b/tests/qemu-iotests/301.out
@@ -2,13 +2,7 @@ QA output created by 301

 == qcow backed by qcow ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=33554432
-qemu-img: warning: Deprecated use of backing file without explicit backing format (detected format of IMGFMT)
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=33554432 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
-image: TEST_DIR/t.IMGFMT
-file format: IMGFMT
-virtual size: 32 MiB (33554432 bytes)
-cluster_size: 512
-backing file: TEST_DIR/t.IMGFMT.base
+qemu-img: TEST_DIR/t.IMGFMT: Backing file specified without backing format
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=33554432 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 image: TEST_DIR/t.IMGFMT
 file format: IMGFMT
@@ -36,13 +30,7 @@ cluster_size: 512
 backing file: TEST_DIR/t.IMGFMT.base

 == qcow backed by raw ==
-qemu-img: warning: Deprecated use of backing file without explicit backing format (detected format of raw)
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=33554432 backing_file=TEST_DIR/t.IMGFMT.base
-image: TEST_DIR/t.IMGFMT
-file format: IMGFMT
-virtual size: 32 MiB (33554432 bytes)
-cluster_size: 512
-backing file: TEST_DIR/t.IMGFMT.base
+qemu-img: TEST_DIR/t.IMGFMT: Backing file specified without backing format
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=33554432 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=raw
 image: TEST_DIR/t.IMGFMT
 file format: IMGFMT
