G4S faces continued pressure over the Olympics security debacle, as share prices continue to fall.

On Tuesday bosses at the private security firm insisted they would be claiming tens of millions of pounds in management fees despite admitting being 100% responsible for a "humiliating shambles".

By the end of Tuesday, G4S shares had fallen 17% since the crisis emerged last Wednesday, wiping £700 million from its market value.

Chief executive Nick Buckles is under pressure to quit his £830,000-a-year job over the fiasco, which has resulted in the emergency deployment of soldiers, marines, airmen and police officers.

Mr Buckles admitted on Tuesday he was sorry and "deeply disappointed" after the firm failed to deliver on its £284 million contract, saying he could not deny the debacle was a "humiliating shambles for the company". But he repeatedly insisted the firm still intended to claim its £57 million management fee for work over the last two years.

Keith Vaz, chairman of the Commons Home Affairs Select Committee, said it was "astonishing" and called on G4S to waive the fee and any others associated with the contract, while other MPs on the committee also criticised the firm.

Meanwhile, on Tuesday retired police officers claimed they were told they were no longer needed to help with security at the Olympics because they were "surplus to requirements". David Anderton, secretary of the Merseyside branch of the National Association of Retired Police Officers, said just days after he was sent an email offering employment at the Games for retired officers, he was sent another from the private security firm saying further recruitment was "surplus to requirements" because of the deployment of military and extra police resources.

G4S said the letter was sent about a week ago at a time when it had been refused dispensation to use former police officers who had not completed necessary training and accreditation, and they were not "surplus to requirements" but lacked necessary accreditation and cannot be employed at London 2012.

The Ministry of Defence (MoD) said that contingency plans are in place to provide additional troops if needed, but added that no request has yet been received. Press reports suggested that a formal request could come from the Government as early as Thursday if G4S fail to deliver enough guards. The MoD would not confirm suggestions that 2,000 troops could be involved.

An MoD spokesman said: "As the Defence Secretary made clear at the weekend, should there be a requirement for additional military personnel the MoD will do whatever possible to make them available. At the present time no further requests have been received but, as people would expect, an ongoing programme of prudent planning continues."