G4S boss Nick Buckles was under pressure on Saturday night 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 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 £50m following its failure to meet its contractual obligations.
Buckles said there would be a penalty charge "in the range" of £10 to £20m, 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.
Vaz also questioned whether the G4S security staff would be up to the job after 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 £86m. 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 £284m.
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.
Vaz: 'This is not a dodgy builders' firm. This is the biggest security firm in the world'
Buckles said it had proved to be a highly complex operation recruiting so many people for a relatively short period of time.
"We are recruiting a large number of people, they are all working through a process of interview, two or three different degrees of training, licensing, accreditation," he said.
"Our review process was around the number of people applying for interviews, we had 100,000 of those, the number of people interviewed which was 50,000.
"So basically you work through that process of numbers and as they were getting ready for deployment over a period of time, it's only when you get closer and closer to the Games that you realise that the number isn't as high as you expect."
He said the company had been "fully transparent" with ministers and Locog about the difficulties it encountered.
"We have been sharing the information with our customers - Locog and ministers - for days now," he said.
"They knew that the process of training and recruiting and vetting had to be compressed over the last seven or eight weeks."
He 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."
During a bruising round of broadcast interviews, Buckles struggled at one point to say whether all the staff they had recruited could speak fluent English, saying it was "difficult question to answer".
"I am pretty sure that they can, but I can't say categorically as I sit here today," he told the BBC Radio 4 Today programme.
Vaz said the competence of the G4S security staff was one of the key issues his committee would want to look at.
"Have they been properly trained? Are they going to be able to communicate with those that arrive at the Games and are they qualified to do what they are supposed to do?" he said.
He expressed concern about the long-term implications for G4S's role as a provider of security services to the public sector.
"This is not a dodgy builders' firm. This is the biggest security firm in the world. They have hundreds of millions of pounds of contracts to deal with policing, prisons, detention centres, which the Home Office has given them. The worry for me is the long term," Vaz said.
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