The decision to deploy the extra troops, who had already been put on standby, was taken at a meeting of the government's emergency committee, Cobra, on Tuesday.

"With three days to go until the opening ceremony, with an incredibly busy weekend, we don't want to leave anything to chance," said Jeremy Hunt, the cabinet minister with responsibility for the Games.

"It's better to have those troops on the ground so that, were they to be needed, they can swing into action immediately," he added, insisting that there was "no additional threat."

"We are very confident that we will do a good job, but there is absolutely no room for complacency and the next couple of days are critical," said Hunt.

The government remained confident that enough numbers were available to "deliver a safe and secure Games."

Hunt denied that the move was linked to the recent fiasco concerning private security contractor G4S, indicating that the problems had been overcome.

The firm had recently stepped up its supply of staff for the Olympic Park. Its failure to provide the full number of 10,400 guards prompted the call-up of 3,500 soldiers last week.

However, during a parliamentary hearing last week, G4S chief Nick Buckles said he could not guarantee that the 7,000 people he hoped to have trained for the job would actually turn up on opening day.

The number of soldiers at the Olympic Park has now risen to 18,200 - of a total security of 40,000.

With the opening ceremony three days away and the football starting Thursday, Olympic organizers LOCOG said the latest troops move would suffice to "deliver the Games in just about any scenario."

The decision came amid concern over a lack of training of G4S staff in operating the airport-style X-ray machines and other security measures.

LOCOG chief executive Paul Deighton described the failure of G4S to fulfil its contract as "a great disappointment," but said the firm had supplied just under 6,000 personnel so far.

"Roving teams" would be deployed to "check guards' work and make sure it is up to scratch," a LOCOG spokesman said.

Meanwhile, Heathrow airport experienced one of its busiest days so far, handling more than 217,000 passengers - including 1,200 athletes and more than 3,000 other Games-related arrivals, the airport said.

Motorists in west London faced long queues as a result of road layout changes because of the Games, with the full stretch of 45 kilometres of lanes reserved for Olympic traffic only opening Wednesday.

There were recurrent signal failure problems on key underground train lines to the Olympic Park, and overground lines suffered overheated cables in the hot weather.

It is estimated that an additional 1 million people will use the Tube network during the Games - which handles an average of 3 million journeys on a normal day.