Brig. Ed Butler, who spent six months commanding British forces in Afghanistan, also revealed fears that militant Islamic groups in south-east Asia are supporting terrorist plots in the UK.
The brigadier, a former head of the SAS, spoke exclusively to the Daily Telegraph in the week when the British death toll in Afghanistan reached 114, with 17 fatalities in the last two months.
UK forces have uncovered evidence that British Muslims are actively supporting the Taliban and al-Qa’eda in attacks on coalition forces in southern Afghanistan, Brig Butler said.
He said: “There are British passport holders who live in the U.K. who are being found in places like Kandahar.”
Earlier this year, it was revealed that RAF Nimrod spyplanes monitoring Taliban radio signals in Afghanistan had heard militants speaking with Yorkshire and Midlands accents.
Privately, British officers in Afghanistan estimate that several thousand Taliban fighters have been killed since 2006, among them people from outside the country.
One officer said: “While my troops have not actually found British passports on enemy dead there has been a suspicion that with the high number of Taliban casualties they have needed to recruit a lot of foreign fighters and some of these are likely to be of British-Muslim descent.”
Disturbingly, Brig Butler suggested the traffic between Britain and Afghanistan may flow in both directions, with some British Muslims returning from the region and posing a domestic security threat.
Brig Butler, 46, said he had seen evidence that terror groups based in southern Afghanistan were plotting with Muslim extremists in Britain to carry out terror attacks in the UK.
“There is a link between Kandahar and urban conurbations in the UK,” said Brig. Butler. “This is something the military understands but the British public does not.”
Western intelligence agencies are increasingly concerned that Afghanistan and its lawless border with Pakistan are now home to many training camps used by Jihadi groups to prepare radicals for attacks in the West.
A Whitehall source confirmed that the security services are aware of some radicalized British Muslims returning to the UK from Afghanstan.
The source said: “There are very small numbers of British citizens traveling out there, being trained up and then returning to the UK.”