Britain's army will supply extra troops to guard the Olympic Games over fears that private security firm G4S will not be able to provide sufficient personnel, the government confirmed on Wednesday.

Responding to media reports that the military had been asked to provide up to 3,500 extra personnel, the country's interior ministry acknowledged it had intervened "by revising the level of military support".

Defence Secretary Philip Hammond would announce details of the new plans to parliament through a written statement on Thursday, the Home Office said.

"We retain the ability to be flexible in our response," the statement 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."

The government insisted it was confident of sticking to the 553 million pounds ($877 million, 662 million euros) budget for venue security.

A security force of more than 40,000 people, backed by a huge intelligence operation, is to guard the Games, which start on July 27.

The Ministry of Defence was due to provide 13,500 troops, but could now be asked to supply 16,500. They will work alongside police, private security guards and unpaid volunteers.

A G4S spokeswoman admitted they had "encountered some issues in relation to workforce supply and scheduling."

"We accept that the government has decided to overlay additional resources and remain committed to keep London 2012 safe and secure," she added, according to the Press Association.

Shadow Olympics minister Tessa Jowell said it was "imperative that action is taken to ensure that the full and necessary quota of security personnel are in place".

"But this is clearly a serious problem, and we have to understand how this problem arose.

"We need to know why the problem has emerged so late in the day and precisely what has been agreed to," she added.