diff --git a/http-core/src/main/scala/org/apache/pekko/http/impl/engine/parsing/SpecializedHeaderValueParsers.scala b/http-core/src/main/scala/org/apache/pekko/http/impl/engine/parsing/SpecializedHeaderValueParsers.scala
index 883cf8ffe..0e0cec6f0 100644
--- a/http-core/src/main/scala/org/apache/pekko/http/impl/engine/parsing/SpecializedHeaderValueParsers.scala
+++ b/http-core/src/main/scala/org/apache/pekko/http/impl/engine/parsing/SpecializedHeaderValueParsers.scala
@@ -33,21 +33,32 @@ private[parsing] object SpecializedHeaderValueParsers {
   def specializedHeaderValueParsers = Seq(ContentLengthParser)
 
   object ContentLengthParser extends HeaderValueParser("Content-Length", maxValueCount = 1) {
+    // The field value is `1*DIGIT`, surrounded by optional whitespace (RFC 9110, section 8.6 and RFC 9112,
+    // section 5). Whitespace within the digits must not be skipped: a value like `1 2` would then be read as `12`
+    // here while another implementation in the request path rejects it or reads it as `1`.
     def apply(hhp: HttpHeaderParser, input: ByteString, valueStart: Int, onIllegalHeader: ErrorInfo => Unit)
         : (HttpHeader, Int) = {
-      @tailrec def recurse(ix: Int = valueStart, result: Long = 0): (HttpHeader, Int) = {
+      @tailrec def skipWhitespace(ix: Int): Int = if (WSP(byteChar(input, ix))) skipWhitespace(ix + 1) else ix
+
+      @tailrec def digits(ix: Int, result: Long, seenDigit: Boolean): (HttpHeader, Int) = {
         val c = byteChar(input, ix)
         if (DIGIT(c)) {
           val digit = c - '0'
           if (result > (Long.MaxValue - digit) / 10)
             fail("`Content-Length` header value must not exceed 63-bit integer range")
-          else recurse(ix + 1, result * 10 + digit)
-        } else if (WSP(c)) recurse(ix + 1, result)
-        else if (c == '\r' && byteAt(input, ix + 1) == LF_BYTE) (`Content-Length`(result), ix + 2)
+          else digits(ix + 1, result * 10 + digit, seenDigit = true)
+        } else if (!seenDigit) fail("Illegal `Content-Length` header value")
+        else lineEnd(skipWhitespace(ix), result)
+      }
+
+      def lineEnd(ix: Int, result: Long): (HttpHeader, Int) = {
+        val c = byteChar(input, ix)
+        if (c == '\r' && byteAt(input, ix + 1) == LF_BYTE) (`Content-Length`(result), ix + 2)
         else if (c == '\n') (`Content-Length`(result), ix + 1)
         else fail("Illegal `Content-Length` header value")
       }
-      recurse()
+
+      digits(skipWhitespace(valueStart), 0, seenDigit = false)
     }
   }
 }
diff --git a/http-core/src/test/scala/org/apache/pekko/http/impl/engine/parsing/RequestParserSpec.scala b/http-core/src/test/scala/org/apache/pekko/http/impl/engine/parsing/RequestParserSpec.scala
index 53ff54e85..d7e903e03 100644
--- a/http-core/src/test/scala/org/apache/pekko/http/impl/engine/parsing/RequestParserSpec.scala
+++ b/http-core/src/test/scala/org/apache/pekko/http/impl/engine/parsing/RequestParserSpec.scala
@@ -670,6 +670,20 @@ abstract class RequestParserSpec(mode: String, newLine: String) extends AnyFreeS
           |abc""" should parseToError(BadRequest, ErrorInfo("Illegal `Content-Length` header value"))
       }
 
+      "with whitespace inside the Content-Length header value" in new Test {
+        """GET / HTTP/1.0
+          |Content-Length: 1 2
+          |
+          |abc""" should parseToError(BadRequest, ErrorInfo("Illegal `Content-Length` header value"))
+      }
+
+      "with an empty Content-Length header value" in new Test {
+        """GET / HTTP/1.0
+          |Content-Length:
+          |
+          |abc""" should parseToError(BadRequest, ErrorInfo("Illegal `Content-Length` header value"))
+      }
+
       "with Content-Length > Long.MaxSize" in new Test {
         // content-length = (Long.MaxValue + 1) * 10, which is 0 when calculated overflow
         """PUT /resource/yes HTTP/1.1