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The London Olympics countdown clock has been undergoing "rigorous testing" to find out why it stopped less than 24 hours after switch-on.

The timer was unveiled at a ceremony in Trafalgar Square on Monday to mark the 500th day before the 2012 Games.

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

A spokesman for the Swiss-based Swatch Group said the company's head engineer had been flown over from Switzerland to investigate the problem.

"The clock is functioning correctly and we have a team of engineers working in both the UK and Switzerland, who are currently carrying out rigorous testing to ensure that we identify the correct problem and deal with it as quickly as possible," he said.

He said engineers would be going through all the systems of the 6.5m (21ft) digital display to try to make sure the problem did not happen again.

The company said the clock, which was started by four British Olympic champions at 1930 GMT on Monday, was developed by its experts and fully tested ahead of its launch.

The Omega London 2012 clock resumed its countdown on Tuesday evening.

The official ticketing website for the Games was also launched on Monday, with fans being given six weeks to apply for the events taking place next year.