Britain should start pulling its troops out of Iraq at the start of May, Sir Menzies Campbell, the Liberal Democrat leader, insisted as he called on the Government to set a clear timetable for ending military involvement in the war-torn state.

He said British forces should be completely withdrawn from Iraq by October under a "framework for withdrawal" outlined to the Commons yesterday as MPs poured scorn on the Government's handling of the invasion and its aftermath.

Sir Menzies made his demand at the start of the first full, Government-led debate on Iraq since 2004. Labour left-wingers had planned to force a symbolic vote of defiance at the end of the day-long adjournment debate. But, in the event, the debate ended without a division.

William Hague, the shadow Foreign Secretary, called on ministers to launch a high-level privy council inquiry into the war, and warned that the Conservatives would force a Commons vote on the issue if ministers did not acquiesce by the end of the parliamentary session.

Sir Menzies said Britain had already handed three out of four districts under British control back to Iraqi forces and could put the city of Basra under Iraqi command by July. He told MPs: "No one can accuse the UK of cutting and running after four years in which we have tried to the best of our ability to fulfil the objectives of the United Nations resolutions.

"For four years, we have endured the stresses and strains of occupation, stresses and strains more directly borne by our armed forces... I don't think it is any longer reasonable or legitimate to ask our armed forces to bear this burden and that is why the process of withdrawal should begin on 1 May and end in October. It is time to go."

But ministers and the Conservatives attacked the Liberal Democrats, arguing it was wrong to set an "arbitrary" date for forces to withdraw. However, the Government faced intense criticism.

Mr Hague said: "We all have to face up to the fact that the situation is now a grim one and a serious one.

"The overthrow of Saddam Hussein was right but the failure to plan for the aftermath has been a tragic mistake and we are now living with the consequences of that."

Margaret Beckett, the Foreign Secretary, condemned setting a timetable for withdrawal as "dangerous and irresponsible". But she insisted that she "remained confident" that Britain would be able to hand over Basra to Iraqi forces in the spring.

Mike Gapes, the Labour chairman of the Commons Foreign Affairs Committee, said: "Our government has to stay in Iraq for as long as necessary... those who call for timetable withdrawals, instant withdrawals, are taking a huge risk."

Frank Dobson, the former health secretary, said British forces should leave Iraq. He said: "I cannot believe that anyone in the House can expect anything but protracted chaos, misery, death and injury for the people of Iraq whenever the occupation forces withdraw.

"There will be no fairy tale ending to the occupation."

Peter Kilfolye, the former defence minister, attacked the Government for waging war on "a false prospectus". He said: "I fear that nobody ever will be held to account."

Another former Labour cabinet minister, Gavin Strang, added: "At last there is a consensus that the situation in Iraq is horrendous. Throughout last year, we were told the coalition was winning, it was just that we were winning more slowly than expected. Earlier this month, we had acknowledgement from President Bush that the situation in Iraq is unacceptable and that existing policies had failed."

Sir Malcom Rifkind, the former Conservative foreign and defence secretary, said: "The reality is that war was a terrible mistake. He should have reflected on the remark of Bismarck that pre-emptive wars are rather like committing suicide because of a fear of death."

Sir Gerald Kaufman, Labour MP for Manchester Gorton, said: "Leaders of the West hailed the democracy involved in the election of the Iraqi government. What has resulted from that election is a vengeful sectarian gang who are hounding their religious opponents."

Michael Ancram, the former Conservative deputy leader, added: "It is time to tell our troops they have, with immense professional courage, done everything we have asked. What is certain is that the escape from the quagmire will not be found in repeating the Vietnam war blunders pouring in more troops."

n Paddy Ashdown, a former Liberal Democrat leader, has contradicted the strategy for pulling British troops out of Iraq put forward yesterday by Sir Menzies Campbell.

Lord Ashdown of Norton-sub-Hamdon, in an interview, indicated that the troops could not be brought home until the rule of law and the economy were rebuilt, or it would hand the country over to criminals.

He agrees with Sir Menzies that countries such as Iran must be brought into negotiating a solution, but unlike Sir Menzies, Lord Ashdown says that fighting wars will be necessary.

US diplomat hints at split with Britain over troop withdrawal

Relations between the UK and the White House over Iraq are under the spotlight after a leading American diplomat appeared to indicate policy splits.

Following comments from the Foreign Secretary, Margaret Beckett, that she was confident of returning Basra to Iraqi control in the spring, allowing up to 7,000 troops to come home, the US ambassador to Iraq Zalmay Khalilzad told the BBC the US would prefer British forces to stay longer.

"We're talking about this. It's clear what our preference would be ­ the longer we stay together here, the better. We would like the British to co-ordinate and for us to have a joint plan," he said, adding that he was confident that a "mutually acceptable" agreement would be reached.

The Ministry of Defence quickly denied any transatlantic "tension" in the wake of last night's remarks by Mr Khalilzad, and insisted it was working "hand in glove" with the US.

But the Tories said that the implications of Mr Khalilzad's comments were "very serious for our troops and for the success of operations in Iraq".

Joe Churcher

The speeches

Sir Menzies Campbell Liberal Democrat Leader

'I don't think it is reasonable or legitimate to ask our armed forces to bear this burden and that is why I believe the process of withdrawal should begin on the first of May'

Frank Dobson Former Health Minister

'I can't believe that anyone in the House can expect anything other than protracted chaos, misery, death and injury for the Iraqi people whenever the occupation forces withdraw'

William Hague Shadow Foreign Minister

'The overthrow of Saddam Hussein was right, but I believe the failure to plan for the aftermath of that has been a tragic mistake and we are now living with the consequences'

Sir Malcolm Rifkind Former Foreign Secretary

'He [Blair] should have reflected on the remark of Bismarck that pre-emptive wars are rather like committing suicide because of a fear of death'