Defence secretary Philip Hammond is expected to announce more troops to boost security at the London 2012 Olympics. Photo: Steve Parsons/PA Wire

Up to 3,500 extra troops will be needed for security duties during the Olympics amid fears that private firm G4S will not be able to meet the needs, the defence secretary will say today.

Philip Hammond will say the military force at the Olympics will be boosted to some 17,000 troops with just two weeks to go.

It comes as security-providers G4S admitted it was experiencing “some issues in relation to workforce supply and scheduling” and had accepted that the government was turning to the military for extra help.

The move, a major embarrassment for organisers Locog, comes as Mr Hammond appears before the Commons defence committee to explain how the Army is to lose 17 major units - including five infantry battalions - in the biggest re-structuring of the service for decades.

The changes - to be completed by the end of the decade - will see the regular Army cut from 102,000 to 82,000 while the Territorial Army will be doubled to 30,000 to give a combined force of 112,000.

Some 17,000 troops will now be involved in the Olympics, with 11,000 of these involved in the security of more than 30 sporting venues and some 70 non-competition venues, including car parks and hotels.

Military personnel will also be involved in specialist support roles including air security, search teams, communications and logistics, among others.

Mr Hammond will give further details to MPs today.

A London 2012 spokesman said: “Security for the Games is big and complex but we have the best brains in the security business working on this - Home Office, Metropolitan Police, Ministry of Defence and world’s largest private security business.

“Delivering a safe and secure Games is everyone’s number one priority.

“We do not anticipate an increase in the overall venue security numbers.”