His admission came as Charles Kennedy, the party leader, said that his party needed to “think afresh” about an exit strategy for Iraq.

The Lib Dems’ stance on the Iraq war was a big vote-winner in the general election and there is now internal party debate over the exact line to take.

In January, Sir Menzies said in a joint statement with the late Robin Cook that British troops should leave by December 31, at the end of the United Nations mandate for Iraqi elections under an agreed constitution. He said yesterday that “we must begin to bring this occupation to an end”.

Later, Mr Kennedy told the conference: “We need to devise a responsible exit strategy. We need to think afresh about the other solutions.”

After his speech, Sir Menzies admitted that “an awful lot of work would need to be done” even for the first troops to start leaving. Speaking to The Times, he said: “What we need is an exit strategy. We do not have one and we must begin to get one. It will take a considerable time to bring 8,000 troops home. You cannot bring about an orderly withdrawal without careful preparation and proper planning.” Sir Menzies also revealed during his conference speech that debate was raging behind the scenes on whether the Lib Dems should support the replacement of Britain’s independent nuclear deterrent.

Mark Oaten, the home affairs spokesman, told the BBC Radio 4 Today programme that there was a strong argument for not replacing the Trident submarine because the cash could be better used elsewhere.

But Sir Menzies told delegates: “The decision as to whether there shall be a successor, in any form, to Trident, will depend on cost, treaty obligations, the strategic environment and threat assessment.”

The Government has said that a decision on whether to replace Trident will have to be taken some time in the current Parliament and John Reid, the Defence Secretary, has promised a public debate.

Mr Oaten said that given the need to carry out more peacekeeping-style operations — they should take the opportunity to consider whether it would be better to invest more resources in putting troops on the ground. “There is a strong argument for getting the balance more towards what we have needed in the last ten years, which is troops on the ground with proper equipment, rather than necessarily very expensive machinery — like Trident.”

Lord Steel of Aikwood, the former party leader, said he hoped that the conference would find time to debate the issue of replacing Trident, even though it is not on the main agenda.

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