Tony Blair says he will not change his strategy on Iraq despite increasing doubts from senior figures in both the UK and the US about how it is working.
Mr Blair said it would be 'disastrous' to withdraw prematurely from Iraq
Lib Dem leader Sir Menzies Campbell said it was clear that the government's "strategy has failed... the choice is stark: change the strategy or get out".
Mr Blair told the House of Commons it was right to discuss the strategy - but added that it would not change
"To withdraw prematurely before the job is done would be disastrous," he added.
The exchanges came after British army chief General Sir Richard Dannatt warned that UK troops' presence in Iraq had "exacerbated" Iraq's security problems and should withdraw "sometime soon".
Former US Secretary of State James Baker is compiling a report which, it has been reported, could recommend the large-scale withdrawal of troops from Iraq.
Help or provocation?
Mr Blair said the government's policy was "to withdraw progressively from Iraq" as Iraqi forces built up their capability for taking on the security role being carried out by UK troops.
"If we desert the Iraqi Government now, at the very time when they are building up the forces, so that the Iraqi forces can take over security, it would be a gross dereliction of our duty to them," he said.
Adopting such a policy would only give heart to the extremists, he said.
Conservative leader David Cameron pressed Mr Blair to say whether he now agreed with Sir Richard's assessment of the situation.
Mr Blair replied that it was important to hand over to Iraqi forces when they are ready.
"Otherwise, of course, we are a provocation rather than a help to them," he added.
"That's why earlier this year we ceded control of the Almatana province. There are now 5,000 Iraqi forces in there doing that job.
"We have already reduced our forces significantly over the past few years."
Mr Blair said in Basra, UK troops were working with Iraqi colleagues to clean out the militia and put in place proper Iraqi security forces to carry out reconstruction work.
"I don't want to dismay our allies or hearten our enemies by suggesting we will do anything else other than stay until the job is done," he said.
"I believe it is a strength that there has been a bi-partisan policy on this and I hope that is maintained."