BBC SPORT | Boxing | Aussie steps in for Khan contest

Khan, 21, will defend his Commonwealth lightweight crown against the Australian veteran on 2 February.

"He is certainly a big step up and the toughest opponent Amir has faced in his career," said promoter Frank Warren.

St Clair, the former IBF super-featherweight champion, has never been stopped in a 46-fight career.

But Khan was undisturbed by the change, saying: "I feel in fantastic shape at this point of my training.

"His record looks very impressive and he has been in with some great names like Vivian Harris and Diego Corrales.

"If he has made these two go the distance then I know I've got a hard fight on my hands.

"Obviously, the motivation is to be the first to stop him, which would look fantastic on my record, but I've also got great boxing skills to back it up with.

"There is no way I'm going to let him ruin my year and my world title ambitions."

Khan has his sights set on becoming Britain's youngest world champion since World War Two and he must do it by the end of June to beat the record of former featherweight title-holder Prince Naseem Hamed.

Hamed was 21 years and 230 days when he defeated Steve Robinson for the WBO featherweight title in 1995.

Warren hopes that Khan's fight with St Clair at London's ExCel Arena will be ratified as an eliminator for a WBO title shot.

In his last fight in December, Khan took just 72 seconds to beat former British champion Graham Earl.