Security costs for the London 2012 Olympics could rise to nearly £1bn, but fears of another rise in the overall budget proved false yesterday.

Olympics Minister Tessa Jowell confirmed that an extra £238m of contingency funds would be set aside for security during the games, in addition to a £600m sum previously announced.

She also revealed that the Olympic Delivery Authority (ODA), which oversees construction of Olympic venues and infrastructure, had already been given all the £500m earmarked for initial contingency for costs on early projects.

But she stressed that "months of careful scrutiny" had confirmed that the total cost of hosting the sporting event would remain within the £9.3bn figure which was announced in March.

There was speculation the statement in the House of Commons would reveal that there was a one-in-five chance of the budget rising again.

Costs have already spiralled to nearly four times the estimate of £2.4bn made in 2005 when the city won the right to host the games.

Opposition politicians welcomed the news that costs seemed to have stabilised, but claimed that key pledges remained uncosted.

Yesterday's financial breakdown for the games was the most detailed so far revealed by the government. It followed a request in July by the financial watchdog, the National Audit Office, to draw up a detailed baseline for controlling the budget.

Speaking in the Commons, the Olympics Minister said: "The budget is consistent with the funding package I outlined in March this year. Months of scrutiny have confirmed the ODA has the money to deliver the venues and infrastructure for a terrific summer of sport, as well as leaving a long-term legacy for one of the most deprived parts of the country."

The contingency was confirmed to be enough to cover the risks at both project and overall levels, she added.