Nick Buckles was appearing before the Home Affairs Select Committee's inquiry into Olympics security, after it was revealed last week that G4S had trained only 4,000 of the 13,700 guards it was contracted to supply.
As Event reported,
Keith Vaz MP, chair of the committee, said the problems were "a concerning development so close to the Games".
G4S was contracted by the government in 2010 in a deal now worth £284 million. The committee received a letter from Locog chief executive Paul Deighton dated 16 July, which confirmed the number of guards G4S was asked to supply had increased dramatically in changes to the contract in December 2011.
The letter claims G4S informed Locog that it wouldn't be able to meet this target on 11 July, despite oral and written information to the contrary.
Buckles told the committee G4S's inability to fulfill the contract only became clear at a late stage due to complex elements like accreditation and training of staff. "To get 10,000 people plus on the ground in a very short space of time was a huge logistical challenge," he said.
He told the committee he was accountable for the delivery of the contract and was "deeply disappointed" and "embarrassed" by recent events. He said he regretted taking the contract.
Buckles added that he was supported by the board in believing he was still the right man to lead G4S. "My first priority is clearly to make sure we deliver as many people as we can on this contract," he said.
The House of Commons has called the problems a "fiasco" and Locog says it will consider financial penalties against G4S.