LONDON (AFP) - Former British defence chiefs warned on Monday that parts of southern Afghanistan could fall to the Taleban when British troops leave this year, despite Prime Minister David Cameron recently saying they had accomplished their mission.

Former commander of the elite Special Air Service Richard Williams told the Times that there was already evidence of growing collaboration between Taleban insurgents and Afghan soldiers and politicians in the Helmand Province.

"I will be very surprised if the future governor of Helmand...is not very closely connected to those who we call the Taleban," he told the paper.

"We will end up in a very uncomfortable position, where people will say: 'We've lost nearly 500 guys, most of those were in Helmand, and at the end of it all, we have handed Helmand back to a Taliban-sympathetic governor."

Cameron faced criticism last month for saying that Nato-led foreign troops had accomplished their mission of providing security in Afghanistan, in an echo of former US president George W. Bush's much-derided comments on Iraq in 2003.

Britain currently has around 5,200 troops in Afghanistan, down from 9,000 at the start of 2013, and plans to have no combat troops on the ground by the end of this year.

David Richards, who was chief of the defence staff until last year, doubted the Afghan army could cope once the drawdown was complete, saying the ability to deal with insurgents would "rapidly fall away".

"More importantly, the signal of a loss of confidence in Afghanistan would have a devastating effect on the Afghan economy and, therefore in turn, lead to a breeding ground for militant jihadists to return," he told the Times.