SIR Jeremy Greenstock, Tony Blair's former special envoy, has warned that British and American troops may have to be pulled out of Iraq if its government breaks down and the country descends into chaos.

It is an option which the Prime Minister will surely consider unthinkable because it would mean an early end to his premiership and a legacy which would be disastrously tainted.

He would be remembered as the Prime Minister who supported President Bush's attack on Iraq but failed to clear up the aftermath. The coalition would have succeeded in removing a terrible tyrant in Saddam Hussein, but would have been defeated in its longer-term mission to restore democratic order to Iraq.

Much as we wish they could be, it is not the time to withdraw our troops.

With Iraq appearing to be in increasing danger of splintering into dangerous militias, our departure would be an appalling dereliction of duty. Britain and America have helped to create an almighty mess and we should not even be considering leaving it unresolved.

What we need to see, however, is clearer evidence of a coherent plan for our continued presence. What are our troops' main objectives? Do they need further support? And what is the exit strategy?

If we didn't have such a strategy at the start of the war - and that appears increasingly to be the case - there certainly needs to be one now.