British lawmakers clamored for an explanation Thursday about why the military needs to field more troops to protect the Olympics after a private security contractor that was paid millions to do that failed to recruit enough staff.

The development is considered a major embarrassment for London's Olympic Organizing Committee just two weeks ahead of the games.

Home Secretary Theresa May confirmed the government will deploy an additional 3,500 troops at the London Olympics amid concerns that the firm G4S -- which had been enlisted to provide the bulk of the 13,200 private security guards across 100 Olympic venues -- may not hit its target because of problems recruiting and training staff.

May stressed to lawmakers that the security operation for the Olympics -- which kick off on July 27 -- had been meticulously planned, but said contingency planning had always been necessary amid a constantly changing security environment.

"Concerns have arisen about the ability of G4S to deliver the required number of guards for all Olympics venues," she said Thursday. "We have now agreed that it would be prudent to deploy additional military support."

The move brings the total number of military personnel -- from all three services and including reservists-- contributing to the safety of the games to 17,000 -- dwarfing the 9,500 troops Britain has in Afghanistan at a time when the armed forces are coping with thousands of job cuts.

British lawmakers questioned why the apparent gap in security had surfaced just two weeks before the Olympics.

"G4S have let the country down and we have literally had to send in the troops," said opposition Labour Party lawmaker Keith Vaz, demanding to know when the issue was first identified and if G4S -- which has millions in contracts from the British government -- will suffer financial penalties.

May said ministers had received assurances from G4S over the last few days but the "absolute gap in the numbers was only crystallized" a day ago.

May stressed that military operations elsewhere will not be affected by the additional deployment for the Olympics, and acknowledged that the move will put extra strain on troops during the summer holiday season.

Britain has committed 553 million pounds ($857 million) for venue security, covering arenas in London and other locations across Britain, including a southwest England sailing center and five soccer stadiums.

The massive security operation for the Olympics also will feature 12,000 police, 3,000 volunteers, Typhoon fighter jets, helicopters, two warships and bomb disposal experts.