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 (GFS.L) said it might not be able to supply the 10,400 security guards it had promised as part of a 284 million pounds deal.

"Let me reiterate there is no question of Olympic security being compromised," Home Secretary Theresa May told parliament.

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.

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.

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

Adding to the bumpy build-up to the Games, British mobile operator O2 (TEF.MC) 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.

Britain's Highways Agency also said on Wednesday it would work around the clock to complete road works on a major motorway between London and Heathrow Airport, where repairs to a cracked road have caused major delays to a key Olympic transport route.

OVER CONFIDENT

The news is a also huge blow to the security firm itself, G4S which employs over 657,000 staff working in areas ranging from cash handling to guarding ships from pirates.

During the Games, security guards are needed to provide airport-style checks to search and screen spectators, and can also check vehicles. They will also be responsible for queue management and protecting the perimeters and equipment.

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

Shares in the firm were down 3 percent as analysts questioned whether the embarrassment could hit other parts of the business.

"We understand the Government's decision to bring in additional resources and will work with LOCOG (the organising committee), the military and other agencies to deliver a safe and secure Games," G4S said in a statement.

"We have encountered some delays in progressing applicants through the final stages but we are working extremely hard to process these as swiftly as possible."

Keith Vas, Labour chairman of an influential parliamentary select committee, lambasted the group.

"G4S has let the country down and we have literally had to send in the troops," he said.

James Brokenshire, minister for crime and security at the interior ministry, told BBC radio that issues around delivering the contract had only emerged in the last few weeks with G4S admitting it had a significant problem in the past 24 hours.

A spokesman for Prime Minister David Cameron said G4S should face a penalty if it fails to deliver. "If G4S don't meet the requirements of the contract, there should be consequences to that."

A spokesperson for London's Olympic organisers LOCOG said on Thursday it was not appropriate to go into the details of a commercial contract.

During an impromptu visit to Heathrow early on Monday morning Vas said he was "appalled" at the queues, saying half the passport desks had been unmanned. According to media reports, people queuing earlier this week started slow hand clapping the staff to show their frustration. (Additional reporting by Kevin Liffey, Mo Abbas, Avril Ormsby, Kate Holton, Maria Golovnina; editing by Philippa Fletcher)

• A Permanent Economic Emergency