Escalating security costs for the London Olympics will drive the budget for the 2012 Games over the £10 billion mark, it has been claimed.

Anti-terrorism measures will push the cost of Games security to an estimated £1.5 billion - nearly three times the original estimate - according to insiders.

The claim comes amid continued delays over a comprehensive security plan being devised by the Home Office and the Metropolitan Police.

Extra measures to protect athletes and spectators from an attack include drafting in the Army. Military helicopters will patrol overhead and jets will be on stand-by to protect a no-fly zone over the Olympic stadium in Stratford.

The Ministry of Defence would charge the Olympic authorities for such a deployment. The security operation is expected to be the largest in peacetime Britain with the two-week event classed as a major terrorist target. However detailed planning for policing and security is in its early stages.

Games chiefs are juggling security with the desire to stage a more open and accessible Games than in Beijing where large parts of the city were off limits to citizens.

Concern over Games security were raised at an Olympic fringe meeting of the Conservative Party conference in Birmingham yesterday.

Shadow Olympics minister Hugh Robertson said: "One of the Government's pledges to the International Olympic Committee is that we make the Games safe.

"We have seen repeatedly at previous Games that the security budget is always the one that goes way beyond original estimates. That is why it is so concerning that there is still no costed and fully-scoped security plan and why security is the biggest single concern for the project at the moment."

Mr Robertson said he was concerned that the police might try to strong-arm the Home Office - the government department in charge of security - into paying for security improvements beyond the needs of the Games. Police demands for a new command and control centre to be built by 2012 to serve the Games are thought to have added to delays.

The total bill for the Games stands at £9.3 billion but that is based on security costs of £838 million. If the security bill reaches £1.5 billion as insiders suggest, it would send the budget beyond £10 billion despite pledges from Olympics minister Tessa Jowell and London Mayor Boris Johnson that the limit was £9.3 billion.

Olympics chiefs based their security planning and budget on the Sydney Olympics, but these had to be redrawn after the London bombings of 7 July 2005 - the day after the 2012 bid win.

Assistant commissioner Tarique Ghaffur was placed in charge of 2012 planning but he protested when Commissioner Sir Ian Blair wanted to strip him of his remit. Mr Ghaffur is suspended pending his case for racial discrimination.

An Olympic source said: "There have been massive delays and this has the potential to take the whole thing way over budget."

A Home Office spokesman said: "The Government and its partners in security are committed to the safety and security of the London 2012 Olympics, and a funding envelope of £600 million has been agreed.

"An integrated, costed security plan is being produced, for ministerial agreement by the end of the year.

"This will cover strategic options for a range of security measures, including fraud and financial crime prevention and protection, counter-terrorism, identity assurance and personal protection."