Liberal Democrat leader Sir Menzies Campbell has called for UK troops to leave Iraq by the end of October.
Sir Menzies says a phased withdrawal of British forces should start in May.
But at prime minister's questions Tony Blair dismissed the call, saying that setting an "arbitrary timetable" would send a "disastrous signal" to Iraq.
MPs are currently debating Iraq in the Commons, with critics of British policy in Iraq expected to force a technical vote to show their unhappiness.
Sir Menzies is the only one of the three main UK party leaders to suggest a withdrawal date.
At prime minister's questions Mr Blair said: "For us to set an arbitrary timetable... that we will pull British troops out in October, come what may... would send the most disastrous signal to the people we are fighting in Iraq.
"It is a policy which, whatever its superficial attractions may be, is actually deeply irresponsible."
In reply Sir Menzies said: "If he feels so strongly, he should come to debate these issues. What could possibly be more important than that the Prime Minister should be here to debate the issue of Iraq at a time when British forces are at risk every day in respect of their lives?"
But Mr Blair, who was addressing business leaders on the public services rather than taking part in the debate, pointed out that they were already debating Iraq.
Sir Menzies, whose party is the third largest in the UK parliament and opposed the war, said Britain should carry out a staged withdrawal of UK forces between May and October.
"In May the three provinces, according to the government, will be handed back to the Iraqis - Basra can be handed back sometime between May and July, we can hand over the transport route between Kuwait and Baghdad which we presently protect for the United States.
"We can withdraw the RAF aircraft that are operating out of Qatar and by October we should be able to bring all of our British forces back."
During the debate shadow foreign secretary William Hague returned to Mr Blair's failure to attend, asking: "Where would this House have been in the Second World War if Winston Churchill had only attended when there was a 'turning point'?"
He also said: "We will have to face up to the fact that the situation there [in Iraq] now is a grim one and a serious one."
But Foreign Secretary Margaret Beckett accused the Tories of "double standards". She said Tory former prime ministers Margaret Thatcher and John Major had not attended foreign policy debates.
No leader had put themselves before the "scrutiny of Parliament" more than Mr Blair, she added, and he would report back to the Commons, once Operation Sinbad had finished in southern Iraq.
She also told the Commons that British troops could hand control of Basra to local authorities this spring, but any definitive timetable for withdrawal would be "dangerous and irresponsible"
The government agreed to hold the adjournment debate on Iraq and the wider Middle East after calls from MPs for the chance to debate the situation.
As no substantive motion will be put, MPs will not have a chance to vote for a change in policy. However, opponents of the war may choose to make a symbolic protest at the end of the debate by voting against the Commons adjourning for the evening.
In October, the government saw off a cross-party bid by MPs to force an immediate inquiry to be held into the Iraq war. But Mr Blair has said he has not ruled out holding one in the future.