The latest US diplomatic documents released by Wikileaks contain harsh criticism of the UK military effort in Afghanistan from 2007 to 2009.
The cables say US officials and Afghan President Hamid Karzai believed UK forces were not up to the task of securing Helmand province on their own.
A US general, Gen Dan McNeill, is quoted as saying that British forces have made a mess of Helmand.
The details have been published in the Guardian newspaper.
Criticism of the British military effort goes back to 2007 when Gen McNeill was the US general in charge of Nato forces.
He was said to be "particularly dismayed by the British effort" and is reported to have said "they had made a mess of things in Helmand, their tactics were wrong".
He criticised a deal with the Taliban which allowed British troops to be withdrawn from Musa Qala in 2006, saying it "opened the door to narco-traffickers in that area, and now it was impossible to tell the difference between the traffickers and the insurgents".
'Not ready to fight'
A cable dated late 2008, from the US embassy, says "we and President Karzai agree that British forces are not up to the task of securing Helmand" without US support.
In another cable, the then Afghan foreign minister, Rangin Dadfar Spanta, expressed disappointment at the ordering of an extra 2,000 British soldiers to Helmand, saying: "They were not ready to fight as actively as American soldiers."
At a meeting with Senator John McCain in December 2008, President Karzai said he was relieved that US marines were being sent to reinforce the British-led mission in Helmand and "related an anecdote in which a woman from Helmand asked him to 'take the British away and give us back the Americans'."
According to the Guardian, criticism of the British operation in Helmand centres on its failure to establish security in Sangin.
Helmand governor Gulab Mangal told a US team led by vice-president Joe Biden in January 2009 that American forces were urgently needed as British security in Sangin was inadequate.
He is reported to have said British troops "must leave their bases and engage with the people".
BBC defence correspondent Caroline Wyatt said these criticisms related to a period in which British commanders themselves were making clear they wanted more troops and resources.
Responding to the latest leaks, a Ministry of Defence spokesman said: "UK forces did an excellent job in Sangin, an area which has always been and continues to be uniquely challenging, delivering progress by increasing security and taking the fight to the insurgency.
"That work is now being continued by the US Marines as part of a hugely increased Isaf presence across the whole of Helmand Province.
"Both Afghan leaders, including the Governor of Sangin, and the US Marines have publicly recognised and paid tribute to the sacrifice and achievements of the UK forces in that area."
A Pentagon spokesman said the contribution and sacrifice of UK troops to the war in Afghanistan was "certainly recognized and appreciated" in the US.
"Marine commanders on the ground in Afghanistan have publicly recognized that British forces did an excellent job in Sangin, an area which has been and continues to be uniquely challenging," he said.
Meanwhile, former defence secretary Geoff Hoon revealed he had written a paper at the time of the Helmand deployment - which took place under his successor John Reid - raising concerns about the manpower available for the operation.
Mr Hoon told The Times that his paper "basically said that we could do this, but only once we had drawn down significant numbers in Iraq".