The plan, obtained by The Times, has been produced in great secrecy over the past month, even though Security Council approval of a “war resolution” hangs in the balance.

The UN is breaking a taboo, and arguably breaching its charter, by considering plans for Iraq’s future governance while it deals daily with President Saddam Hussein’s regime as a legitimate member.

The 60-page plan was ordered by Louise Frechette, the Canadian deputy of Kofi Annan, the Secretary-General, and was drawn up at the UN’s New York headquarters by a six-member pre-planning group. It envisages the UN stepping in about three months after a successful conquest of Iraq, and steering the country towards self-government, as in Afghanistan.

The plan resists British pressure to set up a full-scale UN administration. It also says that the UN should avoid taking direct control of Iraqi oil or becoming involved in vetting Iraqi officials for links to the President or staging elections under US military occupation.

It proposes instead the creation of a UN Assistance Mission in Iraq, to be known as Unami, to help to establish a new government.

UN sources expected the plan to be implemented even if the US goes to war without a UN resolution authorising military action. It recommends that the UN immediately appoint a senior official to co-ordinate its strategy, who would become the UN special representative in post-war Iraq.

Sources said that Lakhdar Brahimi, the UN troubleshooter who organised the creation of the Government in Afghanistan, would be approached about a similar role in Iraq.

Mr Brahimi, a former Algerian Foreign Minister whose journalist-daughter has been reporting for the CNN television network from Baghdad, is said to be reluctant to take on a major assignment at the age of 68 but is expected to accept.

A clause in the UN Charter bars it from interfering in a member state’s internal affairs. When Mr Annan wanted to discuss contingency plans for war-time humanitarian operations with the Security Council last month, Russia insisted that he do so informally in his own office rather than in the council chamber.

Yet Ms Frechette had a 90-minute meeting on Monday with Jay Garner, the retired US Army general who is in line to be the US governor of postwar Iraq.

Lieutenant-General Garner heads the Pentagon office of reconstruction and humanitarian affairs formed in January, which is assembling a “government-in-waiting” of Iraqi exiles and American advisers to head Iraq’s major ministries and public works agencies.

Although the US plans to take control of Iraq immediately after a war, diplomats say that Washington is now more prepared to accept an international role there later on.

General Garner told Ms Frechette that he wanted to get out of the job “as quickly as possible” to be replaced by a respected international figure. He foresaw Iraqi exiles in the transitional administration being replaced in one to six months. “Everyone can swallow up to three months of US government in Iraq,” one UN official said.

The UN plan predicts that, despite the acrimonious divisions in the Security Council, it will inevitably be called on to play a role in postwar Iraq.

“The considered opinion of the pre-planning group is that, while public statements assert that the coalition forces will be responsible for military and civil administration in the immediate period following the conflict, the likelihood of a more substantial involvement of the UN in the transition (post-three month) phase cannot be discounted,” the document says. “As the extent of coalition force control becomes apparent, the Security Council and, indeed, members of the coalition forces may feel that UN involvement may be welcome in certain areas.”

UN sources say that Britain, which is loath to occupy Iraq because of its colonial history there, pushed for a full-blown UN administration along the lines of those in Kosovo and East Timor, and a UN agency to control Iraq’s oil.

But UN planners insisted on respecting Iraq’s sovereignty and said that it could not run a country 33 times the size of East Timor. The document says: “The group found that, although a UN-led transitional administration may seem more palatable than an administration by the occupying power, there are key drawbacks to a transitional administration: the UN does not have the capacity to take on the responsibility of administering Iraq.”

Instead, the UN favours a political process like that in Afghanistan, where Mr Brahimi worked with US officials to organise the Bonn conference of prominent Afghans to set up an interim government.

“The preferred option for the UN is a UN assistance mission that would provide political facilitation, consensus-building, national reconciliation and the promotion of democratic governance and the rule of law,” the plan says. “Full Iraqi ownership is the desired end-state whereby a heavy UN involvement is unnecessary. The people of Iraq, rather than the international community,should determine national government structures, a legal framework and governance arrangements.”

• Some 75 per cent of Britons now back war on Iraq if it has UN approval, according to a Mori poll of 985 people.

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