diff --git a/src/serde/de.rs b/src/serde/de.rs
index db71213..f25ed02 100644
--- a/src/serde/de.rs
+++ b/src/serde/de.rs
@@ -1152,6 +1152,13 @@ macro_rules! deserialize_numeric_key {
         where
             V: de::Visitor<'de>,
         {
+            // The key text must be the number itself: `deserialize_number` skips leading
+            // whitespace, which is not part of a numeric key (`{" 1": 0}` is not key 1).
+            match self.de.parser.read.peek() {
+                Some(b'0'..=b'9' | b'-') => {}
+                _ => return Err(self.de.parser.error(ErrorCode::ExpectedNumericKey)),
+            }
+
             let value = tri!(self.de.deserialize_number(visitor));
             if self.de.parser.read.next() != Some(b'"') {
                 return Err(self.de.parser.error(ErrorCode::ExpectedQuote));
diff --git a/src/serde/mod.rs b/src/serde/mod.rs
index 1a90144..ec9e1e8 100644
--- a/src/serde/mod.rs
+++ b/src/serde/mod.rs
@@ -767,6 +767,41 @@ mod test {
         assert_eq!(collect("[1] [2"), vec![Ok(crate::json!([1])), Err(())]);
     }
 
+    #[test]
+    fn test_numeric_key_rejects_leading_whitespace() {
+        // A numeric map key is the number itself; whitespace inside the quotes is not skipped
+        // (serde_json rejects these too). It used to be: `{" 1": 2}` gave the key 1.
+        for json in [
+            r#"{" 1":2}"#,
+            r#"{"  -1":2}"#,
+            "{\"\t1\":2}",
+            "{\"\n1\":2}",
+            r#"{" 1.5":2}"#,
+        ] {
+            assert!(
+                crate::from_str::<BTreeMap<i64, u8>>(json).is_err(),
+                "i64 key: {json:?}"
+            );
+            assert!(
+                crate::from_str::<BTreeMap<u32, u8>>(json).is_err(),
+                "u32 key: {json:?}"
+            );
+            assert!(
+                crate::from_str::<BTreeMap<i8, u8>>(json).is_err(),
+                "i8 key: {json:?}"
+            );
+            assert!(
+                serde_json::from_str::<BTreeMap<i64, u8>>(json).is_err(),
+                "serde_json: {json:?}"
+            );
+        }
+        // as a string key it is fine, and plain numeric keys are unchanged
+        let m: BTreeMap<String, u8> = crate::from_str(r#"{" 1":2}"#).unwrap();
+        assert_eq!(m.get(" 1"), Some(&2));
+        let m: BTreeMap<i64, u8> = crate::from_str(r#"{"1":2,"-3":4}"#).unwrap();
+        assert_eq!(m, BTreeMap::from([(1, 2), (-3, 4)]));
+    }
+
     #[test]
     fn test_utf8_lossy() {
         let data = [&[b'\"', 0xff, b'\"'][..], br#""\uD800""#, br#""\udc00""#];
diff --git a/src/value/de.rs b/src/value/de.rs
index 5694482..c891d26 100644
--- a/src/value/de.rs
+++ b/src/value/de.rs
@@ -213,6 +213,11 @@ macro_rules! deserialize_numeric_key {
                 _ => return Err(Error::syntax(ErrorCode::ExpectedNumericKey, b"", 0)),
             }
             let number = tri!(de.$using(visitor));
+            // the whole key must be the number: `{"1x": 0}` is not key 1 (as in serde_json, and
+            // as when deserializing from text, where the closing quote must follow the number)
+            if de.parser.read.peek().is_some() {
+                return Err(Error::syntax(ErrorCode::ExpectedNumericKey, b"", 0));
+            }
             Ok(number)
         }
     };
@@ -746,4 +751,37 @@ mod test {
             assert!(from_value::<E>(&v).is_err(), "{bad}");
         }
     }
+
+    #[test]
+    fn test_numeric_map_key_from_value_must_be_whole_number() {
+        // A numeric key is the whole key text. It used to be parsed as a prefix, so
+        // `{"12abc": 1}` gave the key 12 through `from_value` (the text path rejects it).
+        use std::collections::BTreeMap;
+
+        use crate::{from_str, from_value, Value};
+
+        for json in [
+            r#"{"12abc":1}"#,
+            r#"{"1 ":1}"#,
+            r#"{"1,2":1}"#,
+            r#"{"0x1":1}"#,
+            r#"{"08":1}"#,
+        ] {
+            let v: Value = from_str(json).unwrap();
+            assert!(
+                from_value::<BTreeMap<i64, u8>>(&v).is_err(),
+                "i64 key: {json}"
+            );
+            assert!(
+                from_value::<BTreeMap<u32, u8>>(&v).is_err(),
+                "u32 key: {json}"
+            );
+            assert!(from_str::<BTreeMap<i64, u8>>(json).is_err(), "text: {json}");
+        }
+        let v: Value = from_str(r#"{"12":1,"-3":2}"#).unwrap();
+        assert_eq!(
+            from_value::<BTreeMap<i64, u8>>(&v).unwrap(),
+            BTreeMap::from([(12, 1), (-3, 2)])
+        );
+    }
 }