ASHLEY Williams insists referee Lee Probert must stand strong in Saturday's South Wales derby.

The Swansea City defender says a firm hand is vital in order to avoid the controversial events of last season's clash at Ninian Park.

Swansea were on the verge of a dramatic victory over their Welsh rivals following Joe Allen's late strike, only for Ross McCormack to level from the spot in injury time after Williams was adjudged to have fouled Jay Bothroyd.

The Wales defender was inconsolable, and television replays suggested referee Mike Dean had got it wrong.

And seven months on from one of the most painful memories of his career, Williams stresses that a strong display from the match official will be crucial in avoiding a repeat when Cardiff arrive at the Liberty Stadium on Saturday.

Wiltshire official Probert, in the spotlight already this week, has been assigned for the derby.

Probert, criticised for ordering Arsene Wenger to be sent to the stands in front of Manchester United fans earlier this season, wrongly booked Stephen Warnock in Aston Villa's draw at Everton on Saturday.

That yellow card has since been rescinded, while he also dismissed Everton's Diniyar Bilyaletdinov before wrongly sending off Carlos Cuellar for his second caution in the same match.

Williams believes the referee has to be at the top of his game for the biggest game of the season for the Welsh rivals.

"There is going to be a strong ref for this game and that is crucial," said the centre-back. There are so many incidents in these games, and the major calls need to be correct.

"You need the ref to be sensible and be sure of his decisions, otherwise it will spoil it, like in the last derby game.

"It was such a big game and to finish it on that decision was a shame because it ruined a big match. I've not spoken to anyone who thinks it was a penalty.

"I was totally gutted not just for myself, but the players, fans and coaches. I stood my ground and when the ref gave the penalty my heart sank.

"It still grates on me. It was not a penalty — if anything, I should have had a free-kick.

"They do get a lot of penalties, for whatever reason, so we have to be aware of it."

The 25-year-old, who has been part of a Swansea defence that has conceded just two goals in the last nine games, is relishing the clash between two in-form teams.

Williams sampled his first taste of the rivalry during Swansea's Carling Cup victory over Cardiff at the Liberty last September.

And he is calling for a repeat effort from the Swansea support in what he recalls as the best atmosphere he has experienced.

"It was the best game of the season when we beat them in the Carling Cup," said Williams. "It was the most memorable and enjoyable for me.

"The Liberty was bouncing that night. It was the best atmosphere I have ever been in — and that includes internationals. If we can get a repeat of that then it will definitely lift the players."

Cardiff City striker Ross McCormack fired a warning to the Swans yesterday when he scored a spectacular 25-yard free-kick to help Cardiff's reserves to a 2-0 win against Exeter City. McCormack will lead Cardiff's attack on Saturday in the absence of the banned Michael Chopra.