THE head of the private firm at the centre of the Olympics security fiasco yesterday admitted he only realised just over a week ago that the task of supplying guards had been "underestimated".

Nick Buckles, chief executive of G4S, said he was "very sorry" that 3500 troops had to be drafted in at the last minute to make up the shortfall – some of whom have been forced to cancel leave and holidays after operational tours in Afghanistan.

Speaking publicly for the first time since details of the botched security operation emerged, Buckles said: "We accept that we underestimated the task of supplying staff for the Olympics. We deeply regret that."

The problem will cost G4S up to £50 million, including penalty charges and the cost of having to pay the Ministry of Defence for providing additional troops.

G4S was initially contracted to provide 2000 staff for £86 million but, following a review of security requirements last year, that was increased to 10,400 personnel while the value of the contract more than trebled to £284m.

However, last Wednesday, with just over two weeks to go until the opening ceremony of the Games, it emerged the firm was not going to be able to provide the required number of staff and additional troops would be required.

Despite having signed the initial contract in 2010, Buckles said that he had only realised "eight or nine days ago" that there would be a shortfall.

"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 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 [ready to deploy] isn't as high as you expect."

Buckles was also unable to say whether all the staff they had recruited could speak fluent English, saying it was a "difficult question to answer".

"I am pretty sure that they can, but I can't say categorically as I sit here today," he added.

He disclosed that G4S now faced a penalty charge "in the range" of £10m to £20m for failing to fully honour the terms of its £284m contract with Olympic Game organisers, Locog.

But the bulk of the losses – estimated at between £30m and £50m – will come from having to pay the Ministry of Defence for the costs of providing the additional troops.

Buckles also apologised directly to the troops involved, saying: "Thanks very much for supporting us. We're very sorry that you've had to get involved at this late stage, but we're very grateful."

He revealed that his total pay a year was £1.2m, adding he was worth it "99% per cent of the time".

Asked whether he would get a bonus this year, he replied: "It doesn't look like it, does it?"

Buckles, who will face a grilling on Tuesday by members of the House of Commons Home Affairs Select Committee, insisted the company had kept both the government and Locog fully informed about what was going on.

Labour MP Keith Vaz, chairman of the committee, said the problems that had arisen raised further questions as to whether the staff G4S had managed to recruit were up to the job.

Vaz said: "We do need to examine what is the workforce that is in place at the moment. 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?

"I hope this is an opportunity for Mr Buckles on Tuesday to come out with the facts, to be very clear, to be very open, to be very transparent with Parliament so that we can get to the bottom of what has happened."

VAZ also said the difficulties could have implications for the firm's long-term relationship with 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," he said.

However, the MP stopped short of calling for Buckles to resign, saying: "I think we need to wait and see what he says on Tuesday."

Shadow Olympics minister Tessa Jowell said that while there were "hundreds of questions" to be asked about what had gone wrong, the post-mortem should wait until after the Games were over.

"With less than two weeks to go, the focus now has got to be on nailing this down and ensuring the integrity and resilience of the security plan," she said. "I think the questions and scrutiny of how we got to this position come after."

Yesterday, it emerged that troops could be drafted in to help with security at Olympic football matches at Glasgow's Hampden stadium. Eight games are due to take place, with the first to be held on July 25. Strathclyde Police have said they are ready to be called on to help should it be required.

On Friday, Deputy First Minister Nicola Sturgeon wrote to Home Secretary Theresa May demanding reassurances over security at the football events in Glasgow.

The letter stated: "We would be grateful for a firm and immediate assurances that any individual being employed at Hampden will be suitably qualified and trained."