The chief executive of the security firm G4S has admitted he only discovered that his company would be unable to provide adequate numbers of security guards for the Olympics "eight or nine days ago".

Nick Buckles said G4S would lose £30m-£50m on its Olympic contract because of its failure, which will require 3,500 troops to be used to fill gaps in security.

The chief executive also told the BBC's Radio 4 Today programme he could not confirm that all G4S guards were fluent in English.

"That is a difficult question to answer. They all have a right to work in the UK and have been vetted to very high standards. I cannot say categorically as I sit here today [that all the guards speak English]," he said.

Buckles said there were roles in which fluency in English was not a necessity. "I'm saying there probably are roles within the security structure where there is not a direct communication with the public," he said.

Buckles later clarified his replies by saying that all Olympic security guards must be proficient in English to receive their Security Industry Association licence.

The chief executive added: "The workforce that we are recruiting for the Games come from different backgrounds. Some are in current employment so clearly they are not going to be available for training until quite close to the Games and others are students that clearly aren't available until closer to the Games."

Keith Vaz, chairman of the home affairs committee, which has summoned Buckles to appear before them next week to account for the Olympic security fiasco, said the problems raised questions as to whether the staff G4S had managed to recruit were up to the job.

"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?" the MP told BBC News.

Vaz added that the situation raised questions over the firm's government contracts.

"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.

The security firm had been committed to supply 13,700 guards as demanded in its contract, worth £284m, which it now says it is unlikely to be able to fulfil. Overall, the 23,700-strong security force for the Games will include a mix of military, private security guards and at least 3,000 unpaid volunteers. Buckles said it would cost G4S between £314m and £324m to fulfil its contract and pay penalties.

Contingency plans have also been drawn up for police officers to secure venues in the days leading up to the start of the Games because of G4S's recruitment shortage. The original plan was for specialist police teams to search the venues before lockdown was carried out by either G4S or the military.

But plans are now being made for the potential need to deploy hundreds of officers to control what goes into the arenas until the extra military personnel arrive.

Assistant Commissioner Chris Allison, head of Scotland Yard's security operation, said: "Delivering a safe and secure Games is a priority but we will not compromise on keeping the streets and our local communities safe."

The home secretary, Theresa May, had been pressing G4S to provide assurances over its commitments, but patience in Whitehall ran out last weekend and talks began to establish whether the Ministry of Defence could fill the gap.

As May came under sustained opposition attack in parliament this week, she repeatedly refused to spell out what penalties G4S would face, insisting that the contract was with Locog and not the Home Office.

The depth of the crisis over G4S's Olympic security preparations became increasingly clear on Thursday as recruits revealed to the Guardian details of a "totally chaotic" selection process and police joined the military in bracing themselves to fill the void left by the private security contractor.

Opposition politicians have called for a pause in the award of any more government contracts to G4S, and there had been speculation in recent days that it would face financial penalties amounting to tens of thousands of pounds per venue per day for failing to deliver sufficient staff.

Locog, which has also been summoned to appear before the home affairs select committee along with the Home Office and the Department for Culture, Media and Sport, admitted in December it had wildly underestimated the number of staff required to deliver security at 34 Olympic venues in London and around the country.