Labour leader Ed Miliband wants G4S to be barred from taking on any more Government contracts until its competence has been reviewed in the wake of the Olympic security scandal.

Mr Miliband will use a speech to demand a moratorium on any new public money being awarded to the giant firm after what he said was a failure that "beggared belief". And he will present the fiasco as evidence that the coalition should halt moves to massively increase private sector involvement in policing, warning it will undermine forces' resilience.

The under-fire security giant is already involved in £600 million worth of private work for the Home Office alone and had been hoping to further increase its share of the market.

But Mr Miliband, addressing a gathering of his party's candidates to be the first elected police and crime commissioners (PCCs) in England and Wales, will say that ambition must be put on hold. "Before they are awarded any new policing contracts, we need a review of G4S's ability to deliver," he will tell them.

His call comes amid reports that a request may be made for up to 2,000 additional troops to plug any further gaps in security if G4S's problems become more severe. The Ministry of Defence, which has already upped its contribution by 3,500 to 11,000, said that contingency plans were in place to increase the numbers again if necessary.

David Cameron pledged on Wednesday to "go after" G4S over the Olympics security chaos as the Government targeted the firm's multimillion-pound management fee.

G4S boss Nick Buckles made an humiliating appearance before MPs on Tuesday during which he admitted that there would be financial consequences from the company's failure to recruit enough staff but insisted it wanted to keep a £57 million management charge. But sports minister Hugh Robertson said the Government was activating "all penalty clauses" in the contract, some of which applied to the management fee.

Both Mr Cameron and Mr Miliband have been keen to stress that they fully expect the Games to be a major success despite the problems as the row continues to overshadow the final build-up.

Culture Secretary Jeremy Hunt told Daybreak there was a "very strong, robust contingency plan in place" for security at the Olympic Games, and the Government was "not dependent" on G4S.

He added: "I totally reject the suggestion that we haven't had a grip on this because we've been monitoring the contract carefully. They were assuring us right until last week that they were going to meet their requirements. I think they probably didn't know what was going on in their own organisation."