Sir Menzies Campbell has backed calls by military chiefs for British forces to pull out of Iraq because they can "achieve nothing" by staying.
The Liberal Democrat leader has written to Gordon Brown urging him to set a "framework for the complete withdrawal of all our forces" from Iraq.
Sir Menzies says withdrawing from Iraq would make it easier for British forces in Afghanistan, where troops are at "full stretch" in dealing with the Taliban.
"What is being achieved by the continuing British presence in Iraq?" Sir Menzies asked in the letter to the Prime Minister. "There is now a clear recognition that the objectives of their mission cannot be achieved."
The pressure on Mr Brown increased yesterday with the disclosure in The Independent on Sunday that senior military commanders have told the Government that Britain can achieve "nothing more" in Basra, the British sector in southern Iraq which has seen a sharp rise in insurgent attacks over the past year.
The Prime Minister reassured US President George Bush at their first summit last month that Britain would not "cut and run" from Iraq. He said he would wait for the report to Congress by the US Commander in Iraq, General David Petraeus in September.
Sir Menzies said that, while US strategy had been reviewed by the Iraq Study group and General Petraeus was reviewing the use of an extra 30,000 US troops in Baghdad, the British Government had failed to review its policy in Iraq. "This Government has no clear strategy in relation to either Iraq or Afghanistan," he said. "Rather than promoting debate regarding our future role in these conflicts, it has been running away."
US fears that Britain will pull out quickly after the Petraeus report were underlined by comments by an adviser to Mr Bush who said British forces in Iraq were facing an "ugly and embarrassing" exit.
Stephen Biddle, who also advises General Petraeus, has concluded that the Iranian-backed Shia forces will claim a victory over the British when they leave. "It will be a hard withdrawal. They want the image of a British defeat. It will be ugly and embarrassing," he said.
His remarks were attacked as "unhelpful" by the Tories but they were seen by opponents of the war as part of the psychological pressure on Mr Brownto avoidthe humiliation ofa fast withdrawal leaving US forcesexposed