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."
