David Cameron has vowed to "go after" G4S for the costs to the public purse it has racked up by failing to deliver on its Olympics security contract.

During a visit to Afghanistan, the Prime Minister also saluted the military for "stepping up to the plate" by filling the gap left by the security firm.

He said: "Let's be clear, if G4S don't fulfil their contract we will go after them for the money to make sure that they help pay for the military personnel that have been brought in."

Mr Cameron launched an impassioned defence of what had been achieved, including delivering the park on time and on budget.

He said: "Let's not call this the wet and soggy Olympics, it is a great moment for our country.

"When it comes to the Olympics we have to be ready for any contingency. We have to make sure they are safe and secure Olympics.

"Be in no doubt whatever it takes to provide a safe and secure Olympic Games, we will do it."

Meanwhile, Sports Minister Hugh Robertson said the Government would activate "all penalty clauses" in G4S's contract to cut the bill for the taxpayer in light of chaotic security arrangements.

But Mr Robertson dismissed calls for G4S boss Nick Buckles to resign immediately, saying the Olympics needed "stability".

Asked at a press conference what money could be recouped from G4S, Mr Robertson said: "We are working through that at the moment."

He added: "What happens to Mr Buckles is a matter for others in a post-Games environment."