Lieutenant-General Sir Rob Fry, the deputy coalition commander and the most senior UK general in Iraq, said a phased withdrawal was likely to begin “in the pretty near future”.

Fry said he believed the incoming Iraqi government would be “extremely keen” to see a withdrawal because this would show it was in charge.

At the same time Mohammed al-Waili, the governor of Basra, said British control of security was preventing the provincial government from purging the security forces of militia members.

He added that a boycott on co-operation with the British had been suspended last Sunday only after British commanders promised agreement on “a timescale for the execution of future plans”.

His comments came as a senior member of the Mahdi army, the militia of the radical Shi’ite cleric Moqtada al-Sadr, claimed it was behind the shooting down of a Royal Navy Lynx helicopter in which five British service personnel died.

He claimed the militia had bought “advanced” weapons from Iran to end British helicopter surveillance flights.

Fry told a Pentagon briefing on Friday that each of the country’s 18 provinces would be examined on its own merits in any decision to withdraw.

“But I would have thought this process could start in the pretty near future,” he said. “This is a process which I think will accelerate over the next 12 to 18 to 24 months.”

The opportunity to announce the start of a phased withdrawal of British troops is likely to be welcomed by both Blair and senior commanders.

Iraqi reaction to last weekend’s crash has increased backbench pressure on the government to start pulling troops out.

Paul Flynn, Labour MP for Newport West, said television pictures of British troops being protected from a mob by Iraqi police had provided evidence of the need for a phased exit.

“Our troops are now in a situation where their main function is to defend themselves,” he said. “The case for remaining there is collapsing.”

The war has also been deeply unpopular with troops who have suffered a drop in morale. Soldiers and officers alike complain that the deployment, in which 109 British servicemen have died, has left them unable to respond adequately to attack for fear of prosecution.

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