The Telegraph reports:
General David Petraeus will press Gordon Brown to increase the number of British troops patrolling the Iraqi border with Iran when he meets the Prime Minister this week.
The US commander in Iraq wants Britain to move a significant proportion of the 5,000 troops garrisoned at Basra airport to cut off the smuggling of Iranian weapons to Shia militias.
But British commanders fear that the move carries a serious risk of embroiling the UK in a war with Iran at a time when they want to withdraw from Iraq.
A former US under-secretary of defence who is now a Pentagon adviser told The Sunday Telegraph that Gen Petraeus would use the meeting to brief Mr Brown on how Iran is stepping up the supply of weapons and the training of insurgents.
"He will argue that action must be taken soon to stop or at least reduce these activities, and that Britain should be a part of this action," the official said. "He will talk about the possibility of increasing security along the Iraqi border with Iran.
"While he will not make the request, he will present the argument that some British forces now being withdrawn from Basra should be transferred to the border security mission."
Last week, at the Americans' request, 350 British troops from 1 Mechanised Brigade began patrolling the border east of Basra and the Shatt al-Arab waterway.
But The Daily Telegraph has revealed that in November about 2,500 of the Basra contingent could be moved out of harm's way across the border into Kuwait, from where they will escort convoys and train Iraqi troops. The move will put Britain further at odds with US commanders.
An adviser to President George W Bush said Britain should think about sending far more troops to the Iranian border instead.
"There are 5,000 troops there," he said. "We want them to stay in Iraq but we also want them to do something useful."
Dan Goure, a Pentagon consultant, said: "Petraeus will be looking for what the British can do to shore up the Iranian border. We are putting a new base there and it's logical we would seek help from our allies."
The move, in the words of an adviser to Mr Brown, leaves the Prime Minister "spinning like a top between the Americans and Richard Dannatt", the head of the Army, who secured a promise from Tony Blair's government that it would not have to fight on two fronts, in Iraq and Afghanistan.
Gen Petraeus will also call on Britain to keep SAS special forces engaged in Iraq and to maintain control of the headquarters unit overseeing southern Iraq. He will ask for a long notice period if, as expected, the bulk of British troops are ordered home early next year.
For their part, the Americans hope Iranian meddling will force British troops to stay in Basra longer.
But a defence insider who has discussed the issue with officials at the highest levels in the Ministry of Defence said: "They are worried that if they do more on the Iranian border there will be nasty incidents for us at the fag end of a campaign and that we could get sucked into a long-lasting conflict with Iran."
A government official acknowledged: "Gen Petraeus is the commander of coalition forces. If he makes a request, then as long as we have troops there it will be hard to ignore."
Gen Petraeus's trip is designed to damp down angry exchanges over the future of Britain's Iraq deployment.
The Sunday Telegraph has learnt that the US commander in Baghdad, Gen Ray Odierno, is furious at British plans for withdrawal and believes that the Defence Secretary, Des Browne, misled him on the reason for the recent pullback from Basra.
An adviser to Gen Petraeus said: "Odierno said: 'If there's one thing I hate more than being lied to by an American politician, it's being lied to by foreigners'. Browne had to come back to him and admit that it wasn't because the job was done but because the Army can't do both [Iraq and Afghanistan]."
Gen Petraeus will hold a press conference on Tuesday to address British concerns and, in his meetings with Mr Brown and defence chiefs, he will "give assurances that the fighting in Afghanistan is not being neglected as the result of developments in Iraq".
In return, Gen Dannatt is expected to discuss leaving some troops in the Basra headquarters. A US senator who has discussed the issue with Gen Petraeus said: "As far as he is concerned, they are staying."