diff --git a/releasenotes.md b/releasenotes.md
index 0a9b199ef..0575fffd1 100644
--- a/releasenotes.md
+++ b/releasenotes.md
@@ -1,5 +1,16 @@
 # C3C Release Notes
 
+## 0.5.5 Change list
+
+### Changes / improvements
+- Disallow multiple `_` in a row in digits, e.g. `1__000`.
+
+### Fixes
+None
+
+### Stdlib changes
+None
+
 ## 0.5.4 Change list
 
 ### Changes / improvements
diff --git a/resources/grammar/c3.l b/resources/grammar/c3.l
index b4cbc4fd0..c2c567f3b 100644
--- a/resources/grammar/c3.l
+++ b/resources/grammar/c3.l
@@ -21,10 +21,10 @@ B64         [ \t\v\n\f]?[A-Za-z0-9+/][ \t\v\n\fA-Za-z0-9+/=]+
 HEX         [ \t\v\n\f]?[A-Fa-f0-9][ \t\v\n\fA-Fa-f0-9]+
 INTTYPE     ([ui](8|16|32|64|128)|[Uu][Ll]?|[Ll])
 REALTYPE    ([f](8|16|32|64|128)?)
-INT         {D}(_*{D})*
-HINT        {H}(_*{H})*
-OINT        {O}(_*{O})*
-BINT        {B}(_*{B})*
+INT         {D}(_?{D})*
+HINT        {H}(_?{H})*
+OINT        {O}(_?{O})*
+BINT        {B}(_?{B})*
 
 %x COMMENT RAW_STRING
 
diff --git a/src/compiler/lexer.c b/src/compiler/lexer.c
index a3638872b..db80addb8 100644
--- a/src/compiler/lexer.c
+++ b/src/compiler/lexer.c
@@ -417,6 +417,11 @@ static bool scan_number_suffix(Lexer *lexer, bool *is_float)
 	}
 	return true;
 }
+
+#define NEXT_AND_CHECK_NO_MULTIPLE_(lexer__) \
+	do { if (next(lexer__) == '_' && prev(lexer__) == '_') { \
+		return add_error_token_at_current(lexer__, "Multiple consecutive '_' are not allowed."); \
+	} } while(0);
 /**
  * Parsing octals. Here we depart from the (error prone) C style octals with initial zero e.g. 0231
  * Instead we only support 0o prefix like 0o231. Note that lexing here doesn't actually parse the
@@ -429,7 +434,8 @@ static bool scan_oct(Lexer *lexer)
 		return add_error_token_at_current(lexer, "An expression starting with '0o' should be followed by octal numbers (0-7).");
 	}
 	next(lexer);
-	while (char_is_oct_or_(peek(lexer))) next(lexer);
+	while (char_is_oct_or_(peek(lexer))) NEXT_AND_CHECK_NO_MULTIPLE_(lexer);
+
 	if (char_is_digit(peek(lexer)))
 	{
 		return add_error_token_at_current(lexer, "An expression starting with '0o' should be followed by octal numbers (0-7).");
@@ -453,7 +459,7 @@ static bool scan_binary(Lexer *lexer)
 		return add_error_token_at_current(lexer, "An expression starting with '0b' should be followed by binary digits (0-1).");
 	}
 	next(lexer);
-	while (char_is_binary_or_(peek(lexer))) next(lexer);
+	while (char_is_binary_or_(peek(lexer))) NEXT_AND_CHECK_NO_MULTIPLE_(lexer);
 	if (char_is_digit(peek((lexer))))
 	{
 		return add_error_token_at_current(lexer, "An expression starting with '0b' should be followed by binary digits (0-1).");
@@ -512,7 +518,7 @@ static inline bool scan_hex(Lexer *lexer)
 		return add_error_token_at_current(lexer, "'0x' starts a hexadecimal number, so the next character should be 0-9, a-f or A-F.");
 	}
 	next(lexer);
-	while (char_is_hex_or_(peek(lexer))) next(lexer);
+	while (char_is_hex_or_(peek(lexer))) NEXT_AND_CHECK_NO_MULTIPLE_(lexer);
 	bool is_float = false;
 	if (peek(lexer) == '.' && peek_next(lexer) != '.')
 	{
@@ -521,7 +527,7 @@ static inline bool scan_hex(Lexer *lexer)
 		char c = peek(lexer);
 		if (c == '_') return add_error_token_at_current(lexer, "'_' is not allowed directly after decimal point, try removing it.");
 		if (char_is_hex(c)) next(lexer);
-		while (char_is_hex_or_(peek(lexer))) next(lexer);
+		while (char_is_hex_or_(peek(lexer))) NEXT_AND_CHECK_NO_MULTIPLE_(lexer);
 	}
 	char c = peek(lexer);
 	if (c == 'p' || c == 'P')
@@ -547,7 +553,7 @@ static inline bool scan_dec(Lexer *lexer)
 
 	// Walk through the digits, we don't need to worry about
 	// initial _ because we only call this if we have a digit initially.
-	while (char_is_digit_or_(peek(lexer))) next(lexer);
+	while (char_is_digit_or_(peek(lexer))) NEXT_AND_CHECK_NO_MULTIPLE_(lexer);
 
 	// Assume no float.
 	bool is_float = false;
@@ -565,7 +571,7 @@ static inline bool scan_dec(Lexer *lexer)
 		if (c == '_') return add_error_token_at_current(lexer, "'_' is not allowed directly after decimal point, try removing it.");
 		// Now walk until we see no more digits.
 		// This allows 123. as a floating point number.
-		while (char_is_digit_or_(peek(lexer))) next(lexer);
+		while (char_is_digit_or_(peek(lexer))) NEXT_AND_CHECK_NO_MULTIPLE_(lexer);
 	}
 	char c = peek(lexer);
 	// We might have an exponential. We allow 123e1 and 123.e1 as floating point, so
diff --git a/src/version.h b/src/version.h
index d67a62d42..97586da73 100644
--- a/src/version.h
+++ b/src/version.h
@@ -1 +1 @@
-#define COMPILER_VERSION "0.5.4"
+#define COMPILER_VERSION "0.5.5"
diff --git a/test/test_suite/expressions/underscore_errors.c3 b/test/test_suite/expressions/underscore_errors.c3
new file mode 100644
index 000000000..e9038815c
--- /dev/null
+++ b/test/test_suite/expressions/underscore_errors.c3
@@ -0,0 +1,15 @@
+fn void main()
+{
+	int a = 1___0; // #error: consecutive
+	int a = 1_0;
+	float b = 1_0.3__4; // #error: consecutive
+	float b = 1_0.3_4;
+	int a2 = 0x1___0; // #error: consecutive
+	int a2 = 0x1_0;
+	float b2 = 0x1_0.3__4; // #error: consecutive
+	float b2 = 0x1_0.3_4;
+	int a3 = 0b1___0; // #error: consecutive
+	int a3 = 0b1_0;
+	int a3 = 0o1___0; // #error: consecutive
+	int a3 = 0o1_0;
+}
\ No newline at end of file