The government has been forced to defend its decision to recruit an additional 3,500 troops to guard Olympic venues, insisting the move was not evidence of a "shambles" amid a series of setbacks to preparations for the Games.

There is concern among London 2012 insiders that neither the organising committee nor the government is prepared to fully grip the security situation and there are fears of a vacuum in decision making.

The International Olympic Committee president, Jacques Rogge, insisted that despite a last-ditch pile up of issues involving transport, security and the mobile phone network supplying the Games, London's preparations would be "fine by the opening ceremony".But he admitted that incoming athletes and officials, expected to start arriving in large numbers on Monday, would be "inconvenienced" by ongoing problems with the M4.

As G4S came under pressure to reveal the penalty clauses in its contract if it failed to provide the 10,400 guards it had promised, it was criticised by current and former staff.

The home secretary, Theresa May, denied opposition charges that the addition of 3,500 soldiers to the 13,500 already promised to guard the Games and aid police amounted to a "shambles".

Members of the public accounts committee said they planned to investigate the terms of the £284m contract under which G4S in December agreed to increase the number of guards it would provide from 2,000 to 10,400.

It is also supposed to be overseeing the recruitment of 3,300 students, as part of a programme called Bridging the Gap, and 2,500 volunteers. But, as of Thursdayyesterday, only 4,000 staff were fully trained and operational. A further 9,000 are in the "final stages" of the training and accreditation process.

Confirming the additional troop deployment in the Commons, May refused to spell out what penalties G4S would face for failing to provide the required number of guards. She insisted the contract was with the Olympic organising committee and not with the Home Office.

The issue of venue security has been a vexed one for London 2012 organisers. Early estimates that 10,000 staff would be required to guard more than 30 venues leaped to 23,700 last year.

That prompted a scramble to find the additional manpower, including a call on 7,000 of the 13,500 troops that the Ministry of Defence initially agreed to supply. Under the new plans, there are likely to be more than 10,000 troops guarding Olympic venues.

Ian Swales, the Liberal Democrat MP who has examined the G4S deal on the PAC, said he planned to raise the issue when it next met.

"What will the ultimate cost be to the taxpayer, to what extent are G4S liable and how will the taxpayer get reimbursed? How much scrutiny will be possible on the contract and the payments under the contract?" he asked.

"The whole thing was a rip off anyway and then to find they are failing to deliver even what they promised makes it an even bigger scandal to my mind. I was sceptical about the original contract, even if it had been delivered in full and highly effectively. But to now find the delivery has been unprofessional compounds the felony."

May acknowledged that some of the 3,500 military personnel who will now be deployed at the Games would have just returned from operations in Afghanistan and elsewhere.

She said they would be guaranteed any postponed leave and would not be out of pocket. A further 12,000 tickets for the Games will be made available to the armed forces, together with 7,000 tickets to the dress rehearsals for the opening and closing ceremonies.

The shadow home secretary, Yvette Cooper, accused May of presiding over "another Home Office shambles" and Keith Vaz, the chairman of the home affairs committee, said: "G4S has let down the country and you have had to literally send in the troops."

Brigadier Alister Davis, the former British army commander in Afghanistan, who has been in charge of the training operation over the past nine months, admitted this week that it had been a "tall order". He added: "Some things are simpler in the desert."

It is believed that lack of discipline and a subsequent high attrition rate among new recruits contributed to the late running of the training programme. A former Kent police sergeant, Robert Brown, who signed up to work for G4S at the Olympics, withdrew his application over fears that the recruitment process was "totally chaotic".

As leaked internal documents showed at the weekend, the security company has struggled with its deployment as people drop out when faced with the reality of the actual pay and conditions of the job. Organisers agreed to a "just in time" approach in an attempt to control costs, which had almost doubled from £282m to £553m.

The speed with which the Home Office is able to conduct background checks is believed to be another factor.

The mayor of London, Boris Johnson, defended the last-ditch addition of 3,500 troops as an inevitable consequence of "slamming down the remaining loose nails".

He said: "G4S are very confident they can get it right but we're perfectly prudent and right to slot the bolt home and have the safest and securest Olympics you can imagine. The general point I would make is that you would expect at this phase to be a general PR trough."

Rogge said last-minute problems were typical in the final weeks before a Games. "It's not peculiar for London, we have always had difficulties in the time leading up to the Games. This is something that doesn't worry us. It will be fine by the opening ceremony," he said.