Olympic Games security: Philip Hammond announcement due

Details of an additional 3,500 troops expected to be provided to help with 2012 London Olympics security are set to be announced by ministers later.

It comes after it emerged that UK armed forces are on standby to provide more troops in addition to 13,500 already agreed amid fears private contractor G4S may not have enough trained staff.

G4S said it had "some issues in relation to workforce supply".

Labour's Tessa Jowell said there was "clearly a serious problem".

The shadow Olympics minister said there was a need to know why the problem has emerged so late in the day and precisely what has been agreed to.

Meanwhile, the chief inspector of borders has raised fresh concerns about the border controls at Heathrow in the run up to the Olympics.

The Home Office said Defence Secretary Philip Hammond would announce more details about the extra troops on Thursday.

It could result in summer leave for some troops is likely to be cancelled.

With little more than a fortnight to go before the Games, it's becoming clear that the armed forces are being asked to step in.

The move could come as an embarrassment to G4S.

Whitehall sources are keen to stress that there will be no negative impact from all this on security at the Games, pointing out that the plan was always to have a mix of civilian and military personnel guarding the Games, with the contingency plans being activated as an "insurance policy".

But there has been anger within the military that they are being asked to step in, not least at a time when the armed forces are being downsized.

G4S is being paid £300m to guard the Games, but the BBC understands it has not been able to guarantee it can supply the 10,000 guards it has been contracted to deliver.

A G4S spokeswoman said the company had accepted "that the government has decided to overlay additional resources".

She added: "This has been an unprecedented and very complex security recruitment, training and deployment exercise which has been carried out to a tight timescale.

"We have encountered some issues in relation to workforce supply and scheduling over the last couple of weeks, but are resolving these every day and remain committed to providing a security workforce for the start of the London 2012 Games."

The armed forces were already due to provide some 13,500 personnel to help at the Games, with more available as a contingency plan.

Whitehall sources are keen to stress that there is no impact on security at the Games.

In a statement, the Home Office said: "We have agreed to offer help to G4S by revising the level of military support."

It added the government and all involved were focused on delivering a "safe and secure Games".

"The government is committing £553m for venue security and we remain confident that we will deliver within budget."

Keith Vaz, the Labour chairman of the Home Affairs Select Committee, has said he will write to the home secretary for an explanation to find out what is happening.

He said he was "deeply concerned" that G4S were now unable to deliver their contract and that "the Home Office have left contingency plans to the last moment".

Ms Jowell said there was "clearly a serious problem".

"It is imperative that action is taken to ensure that the full and necessary quota of security personnel are in place so that the Games will be safe and secure," said the shadow Olympics minister.

Tessa Jowell: "It's a matter of concern that this failure of G4S seems to have emerged"

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

"We also must know whether this affects army commitments elsewhere, which units are providing people and what terms and conditions are given for those who will likely lose periods of leave."

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

The government has previously said a total security force of 23,700 will take care of venue security, including the G4S contingent.

G4S will have overall control of Olympic venue security, working alongside unarmed troops, searching and checking people going into the stadiums and other venues.

Olympic sites number more than 30 sporting venues and more than 70 "non-competition venues" including car parks and hotels.

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