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The digital clock in central London which has been counting down to the 2012 Olympics has stopped, less than 24 hours after it started.

The timer was unveiled in a ceremony in Trafalgar Square on Monday to mark the 500th day before next year's Games.

The display became stuck on 500 days and 7:06:56 for an estimated six hours.

"We are obviously very disappointed that the clock has suffered this technical issue," said a spokesman for the Swiss-based Swatch Group.

The 6.5m (21ft) clock resumed its countdown by Tuesday evening.

"The Omega London 2012 countdown clock was developed by our experts and fully tested ahead of the launch in Trafalgar Square," the spokesperson added.

"We are currently looking into why this happened."

Four British Olympic champions started the clock at 1930 GMT on Monday.

Fireworks went off as rowers Pete Reed and Andy Hodge were joined by sailors Iain Percy and Andrew Simpson.

Coincidentally the same evening saw the start of a satirical ´óÏó´«Ã½ Four comedy, Twenty Twelve, which featured a clock counting the wrong way and then breaking down.

"It could be a case of life imitating art," said a London 2012 spokesman.

"Whilst the clock has stopped, and Omega is trying to sort it out, it does not give us additional time to stage the Games."

The 500-day mark has also seen the launch of the official ticketing website for the Games, with fans now given six weeks to apply for the events they hope to see next year.