diff --git a/php-transformer/src/HtmlToBlocks/Style/EngineSupportCss.php b/php-transformer/src/HtmlToBlocks/Style/EngineSupportCss.php
index 618806958..b023f95e5 100644
--- a/php-transformer/src/HtmlToBlocks/Style/EngineSupportCss.php
+++ b/php-transformer/src/HtmlToBlocks/Style/EngineSupportCss.php
@@ -92,6 +92,13 @@ public function beforeAuthorCss(string $serializedBlocks, string $layoutShellBlo
         if ( str_contains($serializedBlocks, SourceBlockAttributeProjector::SYNTHETIC_IMAGE_FIGURE_CLASS) ) {
             $parts[] = '.' . SourceBlockAttributeProjector::SYNTHETIC_IMAGE_FIGURE_CLASS . '{margin:0}';
         }
+        if ( str_contains($serializedBlocks, SourceBlockAttributeProjector::SYNTHETIC_FILL_IMAGE_FIGURE_CLASS) ) {
+            // The source image fills both axes of its parent. Its core/image
+            // figure is an extra box; give that box the source's full inline
+            // extent so the image's percentage width does not size from an
+            // aspect-ratio-constrained, shrink-to-fit figure instead.
+            $parts[] = ':root :where(figure.' . SourceBlockAttributeProjector::SYNTHETIC_FILL_IMAGE_FIGURE_CLASS . '){width:100%}';
+        }
         if ( str_contains($serializedBlocks, '<video') ) {
             $parts[] = 'video{max-width:100%}';
         }
diff --git a/php-transformer/src/HtmlToBlocks/Style/SourceBlockAttributeProjector.php b/php-transformer/src/HtmlToBlocks/Style/SourceBlockAttributeProjector.php
index 9ddbcef7f..42fcddd3c 100644
--- a/php-transformer/src/HtmlToBlocks/Style/SourceBlockAttributeProjector.php
+++ b/php-transformer/src/HtmlToBlocks/Style/SourceBlockAttributeProjector.php
@@ -17,6 +17,7 @@ final class SourceBlockAttributeProjector
     public const SYNTHETIC_ANCHOR_BLOCK_DISPLAY_CLASS = 'blocks-engine-synthetic-anchor-block-display';
     public const SYNTHETIC_IMAGE_FIGURE_CLASS = 'blocks-engine-synthetic-image-figure';
     public const SYNTHETIC_INLINE_IMAGE_FIGURE_CLASS = 'blocks-engine-synthetic-image-figure-inline';
+    public const SYNTHETIC_FILL_IMAGE_FIGURE_CLASS = 'blocks-engine-synthetic-image-figure-fill';
     public const SYNTHETIC_EMBED_FIGURE_CLASS = 'blocks-engine-synthetic-embed-figure';
     public const CSS_OWNED_INLINE_FLOW_CLASS = 'blocks-engine-css-owned-inline-flow';
     public const CSS_OWNED_LAYOUT_ITEM_CLASS = 'blocks-engine-css-owned-layout-item';
@@ -52,6 +53,8 @@ public function project(
             // so the alignment has nothing left to move.
             if ( $facts->syntheticImageFigureFollowsInlineFlow ) {
                 $attrs['className'] = SourceDom::mergeClassNames((string) ($attrs['className'] ?? ''), self::SYNTHETIC_INLINE_IMAGE_FIGURE_CLASS);
+            } elseif ( $this->sourceImageFillsParent($sourceElement) ) {
+                $attrs['className'] = SourceDom::mergeClassNames((string) ($attrs['className'] ?? ''), self::SYNTHETIC_FILL_IMAGE_FIGURE_CLASS);
             }
         }
         if ( 'core/paragraph' === $name && $facts->isInlineSourceElement ) {
@@ -119,6 +122,21 @@ public function project(
         return ( new EditorListViewContainerNamer() )->apply($name, $attrs, $sourceElement);
     }
 
+    private function sourceImageFillsParent(DOMElement $image): bool
+    {
+        if ( 'img' !== strtolower($image->tagName) ) {
+            return false;
+        }
+        $shape = $this->styleResolver->imageShapeDeclarations($image);
+        foreach ( array( 'width', 'height' ) as $axis ) {
+            $value = $this->styleResolver->resolveCssVariablesInValue((string) ($shape[$axis]['value'] ?? ''), $image);
+            if ( '100%' !== CssValueInspector::comparable($value) ) {
+                return false;
+            }
+        }
+        return true;
+    }
+
     private static function isHiddenAccessibilitySupportElement(DOMElement $element): bool
     {
         $identity = strtolower(SourceDom::attr($element, 'id') . ' ' . SourceDom::attr($element, 'class'));
diff --git a/php-transformer/tests/fixtures/parity/html-linked-fill-image-percentage-height-chain.json b/php-transformer/tests/fixtures/parity/html-linked-fill-image-percentage-height-chain.json
index 0ef600101..fca7b878c 100644
--- a/php-transformer/tests/fixtures/parity/html-linked-fill-image-percentage-height-chain.json
+++ b/php-transformer/tests/fixtures/parity/html-linked-fill-image-percentage-height-chain.json
@@ -65,7 +65,7 @@
     {
       "path": "serialized_blocks",
       "assert": "contains",
-      "value": "<figure class=\"wp-block-image is-resized fill top-crop be-inline-geometry-0920923a5c6cf48a blocks-engine-synthetic-image-figure\"><img src=\"https://example.com/offsite.jpg\""
+      "value": "<figure class=\"wp-block-image is-resized fill top-crop be-inline-geometry-0920923a5c6cf48a blocks-engine-synthetic-image-figure blocks-engine-synthetic-image-figure-fill\"><img src=\"https://example.com/offsite.jpg\""
     },
     {
       "path": "assets.0.content",
diff --git a/php-transformer/tests/unit/section-background-object-fit.php b/php-transformer/tests/unit/section-background-object-fit.php
index 8753e074b..35e7606a9 100644
--- a/php-transformer/tests/unit/section-background-object-fit.php
+++ b/php-transformer/tests/unit/section-background-object-fit.php
@@ -4,6 +4,7 @@
 require dirname(__DIR__, 2) . '/vendor/autoload.php';
 
 use Automattic\BlocksEngine\PhpTransformer\HtmlToBlocks\HtmlTransformer;
+use Automattic\BlocksEngine\PhpTransformer\HtmlToBlocks\Style\SourceBlockAttributeProjector;
 
 $out = ( new HtmlTransformer() )->transform(
     '<style>.section-background img{object-fit:cover;width:100%;height:100%}</style>'
@@ -17,4 +18,42 @@
     exit(1);
 }
 
+$transform = static function (string $html): array {
+    $result = ( new HtmlTransformer() )->transform($html)->toArray();
+    $css = implode("\n", array_map(
+        static fn (array $asset): string => 'css' === ($asset['kind'] ?? '') ? (string) ($asset['content'] ?? '') : '',
+        $result['assets'] ?? array()
+    ));
+    return array( (string) ($result['serialized_blocks'] ?? ''), $css );
+};
+$fillClass = SourceBlockAttributeProjector::SYNTHETIC_FILL_IMAGE_FIGURE_CLASS;
+$fillRule = ':root :where(figure.' . $fillClass . '){width:100%}';
+
+// A source image explicitly fills both axes of its parent. The aspect ratio
+// belongs to the image, while the extra core/image figure needs the parent's
+// full inline extent rather than an intrinsic/shrink-to-fit width.
+list( $fillMarkup, $fillCss ) = $transform(
+    '<style>.media-frame{height:562px}.media-frame img{width:100%;height:100%;aspect-ratio:1440/562;object-fit:cover}</style>'
+    . '<main><div class="media-frame"><img src="landscape.jpg" alt="Landscape" width="1440" height="562"></div></main>'
+);
+if ( ! preg_match('/<figure[^>]*\b' . preg_quote($fillClass, '/') . '\b[^>]*><img[^>]*style="[^"]*width:100%;height:100%/', $fillMarkup)
+    || ! str_contains($fillMarkup, '"aspectRatio":"1440/562"')
+    || ! str_contains($fillCss, $fillRule)
+) {
+    fwrite(STDERR, "FAIL: source-owned fill image needs a full-width synthetic figure without altering native image height\n" . $fillMarkup . "\n" . $fillCss . "\n");
+    exit(1);
+}
+
+foreach ( array(
+    '<style>.media-frame img{width:100%;object-fit:cover}</style><main><div class="media-frame"><img src="wide.jpg" alt="Wide"></div></main>',
+    '<style>.media-frame img{width:100%;height:100%;object-fit:cover}</style><main><figure class="media-frame"><img src="authored.jpg" alt="Authored"></figure></main>',
+    '<main><div><img src="ordinary.jpg" alt="Ordinary" width="800" height="400"></div></main>',
+) as $source ) {
+    list( $otherMarkup, $otherCss ) = $transform($source);
+    if ( str_contains($otherMarkup, $fillClass) || str_contains($otherCss, $fillRule) ) {
+        fwrite(STDERR, "FAIL: full-width figure sizing must require a synthetic figure and source-owned two-axis fill\n" . $otherMarkup . "\n" . $otherCss . "\n");
+        exit(1);
+    }
+}
+
 fwrite(STDOUT, "section background object-fit tests: passed\n");