British military intelligence reports suggest that a small number of UK nationals are among the range of foreign fighters who regularly clash with British troops in Afghanistan, The Telegraph reports.
British military sources said they believe that some of those Britons have been killed fighting against their own country's forces.
The revelation comes amid a growing concern among British military and intelligence officials about militants based in Pakistan launching attacks on British interests in Afghanistan and at home.
Foreign fighters enter Afghanistan from Pakistan's lawless border areas, home to the reconstituted al Qaeda leadership, the paper said.
British commanders in Helmand say they have intelligence suggesting that British Muslims are among the enemies they face, albeit in small numbers.
"We're talking about ones and twos at a time. It's not big numbers, but they are there, definitely," said one officer.
Some of those British Muslims may have been killed in battle with British troops, military sources said.
Confirmation is near impossible, but British troop commanders believe that UK nationals are among the enemy dead.
One officer said: "We can't say for sure. If they don't carry passports, who can you say what nationality a corpse is? But it's a reasonable assumption that we've killed some of them."
Another security source highlighted the case of Rashid Rauf, the Birmingham man wanted by British police in connection with a 2006 plot to bomb transatlantic airliners.
Rauf is believed to have been killed inside Pakistan in a CIA missile attack in November. "He's not the only British Muslim to die out here," said the source.
In August, Brigadier Ed Butler, the former commander of UK forces in Afghanistan, told the Telegraph that there are "British passport holders" in the Taliban ranks.
And earlier this year, it was revealed that RAF Nimrod surveillance planes monitoring Taliban radio signals in Afghanistan had heard militants speaking with Yorkshire and Midlands accents.&13;&13;&13;