London (CNN) -- British lawmakers tore into the chief executive of the security contractor that will be unable to deliver enough guards for the Olympic Games, forcing Nick Buckles to agree Tuesday that the fiasco is "a humiliating shambles."
Buckles, the chief executive of G4S, said the company should not have agreed to provide 10,400 guards for the Olympics, six days after the security giant admitted it could not do so.
"We regret signing the contract," Buckles said under pressure from lawmakers.
The company's failure forced the government to call in 3,500 military personnel to help, and will also require extra police to be deployed.
G4S has a £284 million ($444 million) government contract to provide security staff for the Olympic Games, but only 4,000 guards are trained and ready.
Buckles said there was a company "expectation" that 7,000 will be ready by the time the Games begin on July 27, although he called the exact number "a moving target."
Lawmakers appeared incredulous at the chief executive's assertion that G4S should still claim a £57 million ($89 million) management fee as part of the contract.
"Why?" demanded Keith Vaz, the chairman of Parliament's Home Affairs Committee. "You haven't managed."
Hammered by Vaz for saying he was "disappointed" about the failure, Buckles first said he was "deeply disappointed" and then that he was "sorry."
Labour lawmaker David Winnick then laid into Buckles, insisting several times that the snafu was "a humiliating shambles."
Buckles finally said he could not disagree.
He repeatedly tried to evade a question from lawmaker James Clappison about whether he was confident about the number of guards who would show up on the first day of the Games.
Vaz intervened again, demanding: "You can't give Mr. Clappison the assurance he seeks?"
"I cannot," Buckles said.
The company will reimburse police forces that have to provide officers to cover for G4S shortfalls, and will "consider" paying bonuses to military and police who are called in to help, he said.
Buckles said he was surprised when he got his first indication on July 3 that his company would be unable to fulfill its contract.
The information came "completely out of the blue," he said.
He knew for certain on July 11 the company could not fulfill its contract, he told the committee, which is looking into security for the Games.
The company accepts "100% responsibility" for its failure, and is "extremely grateful to the military and police for helping us out," Buckles said.
Closing the session, Vaz said lawmakers considered the company's performance "unacceptable, incompetent and amateurish" and implicitly suggested Buckles should resign after the Games.
The G4S recruits are supposed to perform tasks including venue perimeter security, such as manning X-ray machines, searching people, searching vehicles and operating closed-circuit television systems, G4S said Sunday.
The Home Office said Monday that the contractor was suffering from a software problem, which meant it could not guarantee who would turn up where and whether guards had the right training.
G4S has not responded to CNN questions about the accusation.
Britain's Department for Transport said Tuesday that the U.S. Department of Homeland Security was sending a small number of Transportation Security Administration officials to Britain for the Games.
The TSA staff "will be based at designated UK airports to act as an onsite liaison for the TSA," the British government department said in a statement.
"The TSA staff will not be conducting security screening operations or security inspections," the statement said.
The Home Office said Monday that the extra immigration staff it had deployed at borders was properly trained, rejecting media reports to the contrary.
Immigration desks have extra staffers, Heathrow said Monday, amid fears of long lines to get into the country.
Retired border officials and retired police officers are among those being brought in to supplement immigration staff, the Home Office said.
Athlete arrivals are expected to peak July 24, with more than 1,200 competitors due on that day.
Heathrow airport expected a record number of passengers on Monday as athletes began to flood into the city for the Olympic Games.
G4S said Saturday that it stands to lose up to $77 million after failing to recruit enough staff.
CNN senior international correspondent Dan Rivers contributed to this report.