Work on the Olympic Stadium was scheduled to begin in July 2008

David Higgins, chief executive of the Olympic Delivery Authority, told BBC Radio 4 that construction would now get under way in April or May 2008.

"The quicker we start the more time we have to ensure completion in an effective time to allow the test events to happen in 2011," he said.

The design of the �496m stadium will be officially unveiled on Wednesday.

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The 80,000-seater venue was originally expected to cost �280m when London set its budget in 2004.

But that figure has risen dramatically, chiefly because of inflation and the fact that VAT was not factored into the original bill.

Higgins added that he was confident there would be no further rise in building costs.