"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.
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 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.
G4S has said that it stands to lose up to $77 million after failing to recruit enough staff.