Olympic security firm calls for back-up as thousands of troops are put on standby

Private contractor G4S warns it may not be able to train 10,000 staff it needs in time

The military has been told it may need to provide an additional 3,500 troops to provide security during the Olympics amid fears the world's largest private security firm may not be able to provide enough trained staff in time.

Thousands of Army troops have been told to prepare for duties at Games sites and given "notice to move" orders – bringing the military presence at the Games to 17,000 to meet any shortfall from G4S, the company charged with providing security for the Games. G4S is contracted to provide 10,000 guards for the London games and is being paid £284m to do so.

But yesterday it admitted that it had experienced "some issues in relation to workforce supply" and was considering whether it could bring in staff from other parts of its business.

The Government has now put an additional 3,500 troops on standby to provide security at the Games. The presence of Games guards dressed in military uniforms is unlikely to go unnoticed by Olympic visitors; the potential for an overbearing security presence has been a talking point in recent weeks. Air-based missile defence systems will sit on civilian roofs and some of the navy's largest battleships are in position in east London and at the sailing venue in Weymouth, Dorset.

G4S said it was planning with the Olympics organising committee Locog and other security agencies to allow for "a variety of contingencies which have been reviewed in the build-up to the Games".

A spokeswoman said the company had accepted "the Government has decided to overlay additional resources." She said the Olympic contract had involved unprecedented and very complex security recruitment, carried out to a "tight timescale".

The Defence Secretary, Philip Hammond, will announce more details about the extra troops today. But Labour's Keith Vaz, chairman of the Commons Home Affairs Select Committee said: "Only on Monday the Home Secretary said she was confident that their partners would deliver a safe and secure games.

"With only 15 days to go until the Olympics we now find out that she has apparently called on the military to cover a shortfall of over 3,500 security guards.

"I am deeply concerned that G4S are now unable to deliver their £284 million contract and that the Home Office have left contingency plans to the last moment."

A Home Office spokeswoman said: "We have agreed to offer help to G4S by revising the level of military support. The focus of the Government and everyone involved is on delivering a safe and secure Games.

"G4S are Locog's lead contractor for venue security and are being supported by the military. Our approach is intelligence-led and risk-based."