G4S boss Nick Buckles is under pressure to explain why he only learned "eight or nine days ago" that his firm was struggling to find the 10,000 security staff it was contracted to provide for the Olympic Games.
Speaking publicly for the first time since details of the fiasco emerged, the chief executive said he was "very sorry" 3,500 extra troops had had to be drafted in at the last minute to make up the shortfall.
But Mr Buckles, who now faces a grilling from MPs, insisted that both ministers and the Games organising committee, Locog, had been kept fully informed of what was happening. "We accept that we underestimated the task of supplying staff for the Olympics. We deeply regret that," he said.
G4S, the world's biggest private security company, is now looking at a loss of between £35 and £50 million following its failure to meet its contractual obligations.
Mr Buckles said there would be a penalty charge "in the range" of £10 to £20 million, although the bulk of the loss would come from having to reimburse the Government for the costs of providing the additional troops.
Keith Vaz, the chairman of the Home Affairs Committee which will question Mr Buckles on Tuesday, warned there could also be long-term consequences for the firm's lucrative relationship supplying services to the public sector.
Mr Vaz also questioned whether the G4S security staff would be up to the job after Mr Buckles was unable to confirm in television interviews that they would be able to speak fluent English.
G4S originally signed a contract with Locog in 2010 to provide 2,000 security guards in a deal worth £86 million. Following a review of security requirements in December 2011, that number was raised to 10,400 while the value of the contract more than trebled to £284 million.
However on Wednesday, with just over two weeks to go until the opening ceremony, it emerged the firm was not going to make the numbers and additional troops would be required.
Mr Buckles also issued an apology to the troops involved - some of whom had been forced to cancel leave and holidays after arduous operational tours in Afghanistan. "Thanks very much for supporting us," he said in a direct message to the troops. "We're very sorry that you've had to get involved at this late stage but we're very grateful."