Khan (right) was making his third Commonwealth title defence

The Commonwealth lightweight champion was taken the distance by the durable Australian, who turned 33 on Saturday.

Khan dominated the early rounds but St Clair was content to absorb his punches and let the 21-year-old tire late on.

But the Bolton boxer showed his stamina and ever-improving ring craft to stay well ahead on the judges' cards and claim victory by a unanimous decision.

All three judges at ringside scored the fight 120-108 in Khan's favour, with St Clair failing to win a round.

However, the former IBF super-featherweight champion has still yet to be stopped or knocked down in his 47-fight career.

Khan said: "He's a strong, world-class opponent and took every shot I threw, but I enjoyed it in there. It was a good learning fight.

"It's another defence, another tough fight. For these fights I always train like a challenger and I think all the training I have been doing as a professional paid off."

Promoter Frank Warren said Khan's next bout could be in America in April, or he may choose to fight for the European title.