Official Olympic timekeeper Omega today confirmed the countdown clock has stopped only a day after it was unveiled in Trafalgar Square.
The clock was the centrepiece of celebrations to mark 500 days to go to the 2012 Games, which it counts down in days, hours, minutes and seconds.
Yesterday's launch was attended by organising committee chairman Lord Coe, Mayor of London Boris Johnson and heptathlon world champion Jessica Ennis among others and today's technical problem is a major embarrassment for Omega.
A statement from the company today read: "We are obviously very disappointed that the clock has suffered this technical issue.
"The Omega London 2012 countdown clock was developed by our experts and fully tested ahead of the launch in Trafalgar Square.
"We are currently looking into why this happened and expect to have the clock functioning as normal as soon as possible."
The clock, which is 6.5 metres high, five metres long and weighs around four tonnes, took 10 people two days to assemble.