The UK and US may have to abandon Iraq if central government breaks down and the country is engulfed by chaos, Tony Blair's former special envoy has said.
There must be a 'reasonable prospect' of success - Greenstock
Sir Jeremy Greenstock said a pullout from Iraq might be needed if the US and UK had no "reasonable prospect of holding it together".
But he said he did not think this had happened yet.
His words follow violent incidents in Basra after the Army rescued two SAS soldiers seized by Iraqi police.
Earlier, Tory leader Michael Howard called for a new strategy to combat insurgency in Iraq.
But he said pulling UK troops out of Iraq could be disastrous and make the country a centre for world terrorism.
In stark contrast, Sir Jeremy said there would be little alternative if it became clear that there was no "reasonable prospect" of holding the country together.
He told BBC News: "It is extremely important that Baghdad and the centre hold a united Iraq together.
"If, as the months go by, this proves to be impossible and Iraq looks as though it is breaking down into a mosaic of different local baronies and militias... then I think the coalition will have to think again about its presence."
Sir Jeremy added: "We should withdraw when either the Iraqi government, properly elected, asks us to do so and sees no further reason for us to stay, or the system becomes so chaotic that we don't have any reasonable prospect of holding it together.
"The latter would be a considerable defeat for the objectives with which we went into Iraq and we must stay there until we are sure that there is no other answer."
Meanwhile, former UK military chief Lord Bramall has also raised questions over UK troops' role in Iraq, saying: "We have got to really have a plan, making up our mind whether there is a job to be done and whether the forces we have got out there at the moment can do it."
These observations have come after a week which has seen authorities in Basra say they will not co-operate with UK troops.
Basra's governor, Mohammed al-Waili, said he would not cooperate with British troops until there was an apology for the raid to free two UK soldiers.
The UK has defended its action, saying the soldiers were handed to militiamen by rogue elements in the police.
Meanwhile, British troops have reduced their presence on the Iraqi city's streets.
Labour sources stress that earlier this week, Iraqi Prime Minister Ibrahim al-Jaafari highlighted his government's efforts to combat infiltration of the security forces.
He said the armed forces and defence and interior ministries had to keep away from political differences.