Britain could be forced to deploy thousands of extra troops in London during the Olympics, after a last-minute security blunder dealt an embarrassing blow to the government just two weeks before the Games.

With the eyes of the world trained on London, the government was forced to answer questions in parliament on the issue on Thursday as it struggled to contain snaking airport queues and extensive road works that threaten to harm its reputation.

About 23,700 security guards had been due to protect venues as part of Britain's biggest peacetime security operation, with 13,500 military personnel already earmarked to contribute.

But on Wednesday, Britain put an extra 3,500 soldiers on standby after the world's biggest security firm G4S said it might not be able to supply the 10,400 security guards it had promised as part of a 284 million pounds ($432 million) deal.

The company had said just last week that it was so confident about its preparations that it thought it could deliver a similar Olympic-sized event elsewhere in the world while at the same time delivering the London Games.

But in a sign of the shambles, worker Neil Durran was recruited by G4S last month and says he is still waiting for the final clearance so he can actually work.

"They've trained me. I've got my CRB (Criminal Records Bureau), I've got my SSA (Social Security Agency) licence, ready to go, [I've] been actually told what to do," he said.

"It is just because they haven't bothered to take the paperwork over to the Home Office, stamp and I can work. So they've got all these people sitting there, all this paperwork and they are not doing anything about it."

A call-up of the additional 3,500 troops would take the tally at the Games to 17,000, more than the 9,500 currently deployed in Afghanistan.

'Bitter pill'

The sudden need for thousands of extra troops means some soldiers who have just returned from Afghanistan will now be thrown into a very different front line.

Patrick Mercer, a conservative MP and a former army officer, says the troops are paying the price for G4S's incompetence.

"They're just back from Afghanistan, literally just back from Afghanistan. Last week they were warned that many of them were going to be made redundant under troop cuts and now they're being told that their leave is going to be cancelled over the summer. Very, very bitter pills to swallow," he said.

The deployment of troops poses an awkward balancing act for the government in trying to guarantee security at the Games, while not letting its precautions become so intrusive that they spoil its atmosphere.

The disclosure follows months of mounting concerns that officials, athletes and fans could have to spend hours queuing to get through London's main Heathrow Airport where border controls have struggled to cope with large crowds.

Thousands of athletes and officials are expected to start arriving into the capital this weekend before the Olympic Village opens its doors on Monday.

And security is not the only problem facing Olympic officials.

Britain has already spent some 9 billion pounds ($13.7 billion) on the Games and much of the early focus will fall on how London can cope with such a massive influx of visitors, four years after a Beijing Games noted for its gleaming new airport and infrastructure.

The main highway from Heathrow airport into London is closed, causing traffic chaos after potentially catastrophic cracks were found in an overpass.

It could have collapsed at any time and while engineers are frantically trying to fix the problem.

They have missed one reopening deadline and there are no guarantees it will be open in time for the games.

Adding to the bumpy build-up, British mobile operator O2 apologised to users on Thursday after a network outage hit hundreds of thousands of customers, raising concern about how it will cope with a jump in demand once the Olympics begin.

ABC/Reuters