Just 24 hours before the Government braces itself for a backbench rebellion over the war against Iraq, Ministry of Defence officials confirmed that the request was being considered positively.

Although the US already has a military base on the island, it can be used only for defensive and training purposes, unless Britain permits attack operations.

The base is a crucial staging post for the B-2 stealth bombers, which are being moved from their home at Whiteman Air Force Base in Missouri. They can be refuelled on Diego Garcia before travelling on to Iraq to undertake bombing missions.

The last time stealth bombers were used for offensive strikes was during the conflict in Afghanistan, when they flew more than 7,000 miles from their US base to bomb Taliban targets.

A number of the bombers will also be based at RAF Fairford in Gloucestershire, another essential staging post on the route to Iraq.

Tomorrow, Jack Straw, the Foreign Secretary, will open the Commons debate on the war against Iraq. It is the first time that Parliament has been able to express its collective opinion on the conflict since the UN Security Council passed the resolution ordering weapons inspectors back to Iraq to ensure compliance with disarmament resolutions.

Although MPs will be allowed to vote on support for the UN resolution, the Government is attempting to block a vote on whether military action should be taken or whether there needs to be another UN resolution.

A number of amendments have been tabled by Labour MPs opposed to military action. Although defeat is highly unlikely, a rebellion of up to 100 MPs would damage the Government's expressed policy that it stands 'shoulder to shoulder' with President George W. Bush.

Bush has made it clear that he does not think a further Security Council vote is necessary to spark a military conflict if Saddam fails to comply with the weapon inspections.

Government whips were this weekend seeking to head off a major rebellion by signalling that they were 'comfortable' with a Conservative amendment that says Saddam should face 'serious consequences' if he does not disarm. By backing a Tory amendment, the Government will split any opposition to its stance.

Although Geoff Hoon, the Defence Secretary, will speak in the debate, MoD officials said that he would be making no substantive announcement on troop deployments ahead of a conflict, which could come as early as February.

'He will take a robust line with those who disagree with the route we are taking, but we are a long way off making public announcements about military deployments,' one official said.

Bernard Jenkin, the Conservative Shadow Defence Secretary, wrote to Hoon over the weekend, saying that, with the threat of action imminent, Hoon should tell Parliament and the public what support Britain intends to offer the Americans.

'Now that the United States has formally requested a contribution from the UK towards military action in Iraq, Parliament must be informed of the Government's policy,' the letter says.

'Reports suggest that you propose to commit up to 30,000 personnel. Parliament, our armed forces and the country need to be told whether these reports are accurate. Given the time necessary to prepare our forces, a decision will need to be taken within the next few days if the UK is to be ready to participate in military operations in the New Year.'

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