The peacekeeping bill following a war against Iraq could top £30 billion a year, an expert analysis has warned.

A report from the International Institute for Strategic Studies said that while an invasion may be swifter and more targeted than in 1991, a military victory this year may be just the first phase of a long term operation to administer and reconstruct the region.

With Gordon Brown having already set aside £1 billion for "international responsibilities" in his pre-Budget report, the chancellor is likely to be asked to find at least an equivalent sum again with the UK's projected cost of war being set at up to £3.5 billion.

This pales in comparison with the estimated cost to America of prosecuting war, which is put at £20 billion.

But while this is less than half of the £78 billion cost of the 1991 conflict, once inflation has been taken into account, the IISS report notes that 90 per cent of president George Bush Snr's bill was picked up by the international community.

In contrast, president George W Bush has yet to receive guarantees other than that from Britain.

Defence secretary Geoff Hoon said on Thursday that the government was prepared to go to war "irrespective of the cost".

Britain is also expected to play a key role in any peacekeeping operation and the Treasury may be asked to find a share of costs which could reach £30 billion a year.

The IISS used estimates based on the costs of policing post-war Kosovo and Afghanistan and says that it is not unreasonable to expect that an occupation of Iraq could last up to five years.

"Bearing in mind that the US occupied Japan for seven years and has had 30,000 troops stationed in South Korea for 50 years, the likely duration of any such force in Iraq is difficult to predict," defence economist Mark Stoker admitted.

"But a five-year occupation by 100,000 troops could add approximately $125 billion to the bill," he said.