diff --git a/json/json.go b/json/json.go
index 9bb7e95..87f14e2 100644
--- a/json/json.go
+++ b/json/json.go
@@ -362,8 +362,25 @@ func writeJSONScalar(ctx *jsonWriteContext, node *yaml.Node) error {
 		ctx.write(quoteJSONString(node.Value))
 		return nil
 
-	case "!!int", "!!float":
-		// Numbers - write as-is
+	case "!!int":
+		// Check for invalid JSON number formats (e.g., leading zeros)
+		// In YAML, "009911" might be parsed as int, but JSON doesn't allow leading zeros
+		// Treat such values as strings to preserve them correctly
+		if hasInvalidJSONNumberFormat(node.Value) {
+			ctx.write(quoteJSONString(node.Value))
+			return nil
+		}
+		ctx.write(node.Value)
+		return nil
+
+	case "!!float":
+		// Check for invalid JSON number formats (e.g., leading zeros)
+		// YAML may tag values like "009911" as float, but they're invalid in JSON
+		// Treat such values as strings to preserve them correctly
+		if hasInvalidJSONNumberFormat(node.Value) {
+			ctx.write(quoteJSONString(node.Value))
+			return nil
+		}
 		ctx.write(node.Value)
 		return nil
 
@@ -384,6 +401,31 @@ func writeJSONScalar(ctx *jsonWriteContext, node *yaml.Node) error {
 	}
 }
 
+// hasInvalidJSONNumberFormat checks if a numeric string would be invalid in JSON.
+// JSON numbers cannot have leading zeros (except for "0" itself or "0.x" floats).
+func hasInvalidJSONNumberFormat(value string) bool {
+	if len(value) < 2 {
+		return false
+	}
+
+	// Handle negative numbers
+	start := 0
+	if value[0] == '-' || value[0] == '+' {
+		start = 1
+		if len(value) < 2 {
+			return false
+		}
+	}
+
+	// Check for leading zero followed by more digits (invalid in JSON)
+	// "0" alone is valid, "0.x" is valid, but "00", "01", "007" etc. are not
+	if value[start] == '0' && len(value) > start+1 && value[start+1] >= '0' && value[start+1] <= '9' {
+		return true
+	}
+
+	return false
+}
+
 // resolveMergeKeys processes YAML merge keys (<<) and returns content with merged values
 func resolveMergeKeys(content []*yaml.Node) []*yaml.Node {
 	if len(content) == 0 {
diff --git a/json/json_test.go b/json/json_test.go
index b6d1628..ab2f868 100644
--- a/json/json_test.go
+++ b/json/json_test.go
@@ -228,6 +228,53 @@ upperCaseBool: FALSE
 `,
 			indentation: 2,
 		},
+		{
+			name:      "integer with leading zeros",
+			yamlInput: `bankCode: 009911`,
+			expectedJSON: `{
+  "bankCode": "009911"
+}
+`,
+			indentation: 2,
+		},
+		{
+			name: "multiple integers with leading zeros",
+			yamlInput: `identifiers:
+  bankCode: 009911
+  sortCode: 001234`,
+			expectedJSON: `{
+  "identifiers": {
+    "bankCode": "009911",
+    "sortCode": "001234"
+  }
+}
+`,
+			indentation: 2,
+		},
+		{
+			name:      "integers with leading zeros in array",
+			yamlInput: `codes: [009911, 001234, 005678]`,
+			expectedJSON: `{
+  "codes": ["009911", "001234", "005678"]
+}
+`,
+			indentation: 2,
+		},
+		{
+			name:      "octal-like integer value",
+			yamlInput: `value: 0777`,
+			expectedJSON: `{
+  "value": "0777"
+}
+`,
+			indentation: 2,
+		},
+		{
+			name:         "quoted string with leading zeros preserved",
+			yamlInput:    `bankCode: "009911"`,
+			expectedJSON: `{"bankCode":"009911"}` + "\n",
+			indentation:  0,
+		},
 	}
 
 	for _, tt := range tests {