A senior United States military adviser has expressed "frustration" at British forces in southern Iraq.
Gen Jack Keane said the police in Basra could not control the violence
Gen Jack Keane, architect of the US "surge", said the British are more focused on training Iraqi troops than controlling "deteriorating" security.
He added that there is a "general disengagement" by UK forces in Basra.
Meanwhile, the US military's deputy commander in Iraq said Britain was on track to hand over responsibility to Iraqi forces in Basra later this year.
Gen Raymond Odierno told the BBC that a final assessment on whether the handover should go ahead would be made next month, but added that the US would not send troops to Basra when the British forces withdrew.
In a report earlier this summer, the International Crisis Group said that Basra was controlled by rival Shia militias, and that British forces there appeared to have given up trying to impose the rule of law.
Retired Gen Jack Keane has just returned from Iraq, where he is acting as a Pentagon envoy.
He told the BBC's Today programme: "I think there is a general disengagement from what the key issues are around Basra.
"I would imagine that is where the source of frustration is.
"The Brits have never had enough troops to truly protect the population and we have found that out painfully in the central region as well."
Asked about the security situation in Basra, he said: "I think it has been gradually deteriorating, with almost gangland warfare and the lack of ability of the police to control that level of violence - so the situation is gradually getting worse.
"The situation will continue to deteriorate."
He added that US commanders wanted to avoid filling any vacuum left by the British.
"That situation could arise if the situation gets worse in Basra if and when British troops leave," he said.
"Now the situation has changed in the south, it is considerably worse, certainly with the kind of gangland warfare that is preying on the people in the south."
He said US leaders were accepting that they needed to expand the number of their ground troops.
Gen Keane added: "I think the same thing applies to the proud and distinguished British Army, it needs to grow in size to help assist in maintaining security as the situations in the 21st century begin to evolve and challenge the West."
A spokesman for the Ministry of Defence said he would not suggest that there were no security problems in southern Iraq, but insisted that the situation in Basra was very different to that in Baghdad.
He added: "There are challenges, but the Iraqis' ability to confront these themselves is growing and it is appropriate that they take on more responsibility for doing so.
"Our plans are not inconsistent with those of the US - we remain united in our strategy of handing over provinces to Iraqi control as and when conditions allow."
Last week General Sir Richard Dannatt, the head of the British Army, said forces were "certainly stretched".
Gen Keane's comments follow a series of critical remarks about Britain's policy in Iraq.