diff --git a/packages/less/lib/less/parser/parser-input.js b/packages/less/lib/less/parser/parser-input.js
index 9a9dc39241..ff33904929 100644
--- a/packages/less/lib/less/parser/parser-input.js
+++ b/packages/less/lib/less/parser/parser-input.js
@@ -214,13 +214,16 @@ export default () => {
      *   group array (or null). `[...]`/`{...}` do NOT shield a reference — only
      *   `(...)` (a declaration-value group) does.
      */
-    parserInput.$parseUntil = (tok, detectBareVar) => {
+    parserInput.$parseUntil = (tok, detectBareVar, stripLineComments) => {
         let quote = '';
         let returnVal = null;
         let inComment = false;
         let blockDepth = 0;
         let parenDepth = 0;
         let bareVarIndex = null;
+        // Open `(` groups, true for those that hold a URL: `url(`, `url-prefix(`, `domain(`.
+        const urlParens = [];
+        let urlDepth = 0;
         const blockStack = [];
         const parseGroups = [];
         const length = input.length;
@@ -278,6 +281,27 @@ export default () => {
                             i++;
                             inComment = true;
                             blockDepth++;
+                        } else if (stripLineComments &&
+                            input.charAt(i + 1) === '/' &&
+                            urlDepth === 0 &&
+                            input.charAt(i - 1) !== ':') {
+                            // A `//` comment, which is not CSS and must not
+                            // reach the output. Two `//` are not comments: one
+                            // inside a URL function, where the value parser
+                            // turns comment absorption off too, so
+                            // `url(//host/x.png)` and `url( //host/x.png)` keep
+                            // their address; and one after a `:`, which is a
+                            // scheme, as in a bare `http://host`.
+                            const before = input.slice(lastPos, i);
+                            if (before) {
+                                parseGroups.push(before);
+                            }
+                            let nextNewLine = input.indexOf('\n', i + 2);
+                            if (nextNewLine < 0) {
+                                nextNewLine = length;
+                            }
+                            lastPos = nextNewLine;
+                            i = nextNewLine - 1;
                         }
                         break;
                     case '\'':
@@ -298,11 +322,16 @@ export default () => {
                         blockStack.push('}');
                         blockDepth++;
                         break;
-                    case '(':
+                    case '(': {
                         blockStack.push(')');
                         blockDepth++;
                         parenDepth++;
+                        const isUrl = /(?:^|[^\w-])(?:url|url-prefix|domain)$/i.test(
+                            input.slice(Math.max(0, i - 11), i));
+                        urlParens.push(isUrl);
+                        if (isUrl) { urlDepth++; }
                         break;
+                    }
                     case '[':
                         blockStack.push(']');
                         blockDepth++;
@@ -314,6 +343,7 @@ export default () => {
                         if (nextChar === expected) {
                             blockDepth--;
                             if (nextChar === ')' && parenDepth > 0) { parenDepth--; }
+                            if (nextChar === ')' && urlParens.pop()) { urlDepth--; }
                         } else {
                             // move the parser to the error and return expected
                             skipWhitespace(i - startPos);
diff --git a/packages/less/lib/less/parser/parser.js b/packages/less/lib/less/parser/parser.js
index d21096cd22..fa7efa04e6 100644
--- a/packages/less/lib/less/parser/parser.js
+++ b/packages/less/lib/less/parser/parser.js
@@ -1874,7 +1874,7 @@ const Parser = function Parser(context, imports, fileInfo, currentIndex) {
              *   prelude (non-value position); accept `@{var}` interpolation and warn
              *   on a bare `@var` reference (which resolves today but is deprecated).
              */
-            permissiveValue: function (untilTokens, deprecateVariables) {
+            permissiveValue: function (untilTokens, deprecateVariables, stripLineComments) {
                 const entities = this.entities;
                 let i;
                 let e;
@@ -1942,7 +1942,7 @@ const Parser = function Parser(context, imports, fileInfo, currentIndex) {
                 }
                 parserInput.save();
 
-                value = parserInput.$parseUntil(tok, deprecateVariables);
+                value = parserInput.$parseUntil(tok, deprecateVariables, stripLineComments);
 
                 if (value) {
                     if (typeof value === 'string') {
@@ -2350,7 +2350,7 @@ const Parser = function Parser(context, imports, fileInfo, currentIndex) {
                 return e;
             },
             atruleUnknown: function (value, name, hasBlock) {
-                value = this.permissiveValue(/^[{;]/, true);
+                value = this.permissiveValue(/^[{;]/, true, true);
                 hasBlock = (parserInput.currentChar() === '{');
                 if (!value) {
                     if (!hasBlock && parserInput.currentChar() !== ';') {
diff --git a/packages/test-data/tests-unit/at-rules-unknown-line-comments/at-rules-unknown-line-comments.css b/packages/test-data/tests-unit/at-rules-unknown-line-comments/at-rules-unknown-line-comments.css
new file mode 100644
index 0000000000..215eb752df
--- /dev/null
+++ b/packages/test-data/tests-unit/at-rules-unknown-line-comments/at-rules-unknown-line-comments.css
@@ -0,0 +1,41 @@
+@supports selector(
+  :focus-visible ) {
+  a {
+    color: red;
+  }
+}
+@supports (display: grid)  {
+  .grid {
+    display: grid;
+  }
+}
+@supports (display: flex) {
+  .flex {
+    display: flex;
+  }
+}
+@supports (background: url(http://example.com/a.png)) {
+  .absolute-url {
+    color: blue;
+  }
+}
+@document url-prefix(http://example.com/) {
+  .doc {
+    color: green;
+  }
+}
+@supports (background: url(//cdn.example.com/x.png)) {
+  .protocol-relative {
+    background: url(//cdn.example.com/x.png);
+  }
+}
+@supports (background: url( //cdn.example.com/y.png)) {
+  .protocol-relative-spaced {
+    color: navy;
+  }
+}
+@supports (a: b) /* keep me */ {
+  .block-comment {
+    color: teal;
+  }
+}
diff --git a/packages/test-data/tests-unit/at-rules-unknown-line-comments/at-rules-unknown-line-comments.less b/packages/test-data/tests-unit/at-rules-unknown-line-comments/at-rules-unknown-line-comments.less
new file mode 100644
index 0000000000..f72f3c0102
--- /dev/null
+++ b/packages/test-data/tests-unit/at-rules-unknown-line-comments/at-rules-unknown-line-comments.less
@@ -0,0 +1,58 @@
+// A `//` comment inside an unknown at-rule prelude is scanned as raw text by
+// $parseUntil, so it used to survive into the CSS and swallow the rest of the
+// line with it.
+
+@supports selector(
+  :focus-visible // a line comment
+) {
+  a {
+    color: red;
+  }
+}
+
+@supports (display: grid) // trailing comment
+{
+  .grid {
+    display: grid;
+  }
+}
+
+@supports (display: flex)// no space before it
+{
+  .flex {
+    display: flex;
+  }
+}
+
+// A `//` inside a URL function, or after a scheme's `:`, is part of an
+// address. These must be left alone.
+@supports (background: url(http://example.com/a.png)) {
+  .absolute-url {
+    color: blue;
+  }
+}
+
+@document url-prefix(http://example.com/) {
+  .doc {
+    color: green;
+  }
+}
+
+@supports (background: url(//cdn.example.com/x.png)) {
+  .protocol-relative {
+    background: url(//cdn.example.com/x.png);
+  }
+}
+
+@supports (background: url( //cdn.example.com/y.png)) {
+  .protocol-relative-spaced {
+    color: navy;
+  }
+}
+
+// Block comments are valid CSS and stay.
+@supports (a: b) /* keep me */ {
+  .block-comment {
+    color: teal;
+  }
+}