British troops could be killed in reprisal attacks after an American soldier allegedly gunned down 16 villagers in southern Afghanistan, the former commander of British forces in Afghanistan said.
Colonel Richard Kemp said the massacre will also cause an erosion of the vital trust allied forces have built up with Afghan civilians over the course of the war.
He told ITV's Daybreak: "One of the most important things that our forces do day-to-day in Afghanistan is to build up trust with the local people and get them to turn against the Taliban and provide extremely important intelligence that enables us to take the Taliban networks apart.
"That trust is going to be eroded by this kind of incident, so not only are we likely to see protests and possibly American, possibly British soldiers killed over what happened on Saturday night, but also a very severe weakening of the relationship between many of the people in Afghanistan who were supporting us."
The former commander said relations between international security forces and the Afghan government would also be weakened as a result of the murders, which took place in two villages close to a US army base in Panjwai, southern Kandahar, on Saturday.
Prime Minister David Cameron's official spokesman said the UK was "sticking to its course" in Afghanistan despite the "tragic event".
Asked if Cameron was concerned about repercussions on British troops, he said: "This was clearly a tragic event. Our hearts and sympathies go out to the families who have died.
"As far as we are aware it was an abhorrent act by a single individual. There is a Nato/Isaf investigation under way.
"But we are sticking to our course and we have a clear plan which is a measured transition which will allow the Afghans to take over responsibility for security from Isaf forces."
Pressed as to the potential for reprisals against UK troops, he said: "We have a defined course and we are sticking to it.
"On the particular issue of UK forces, clearly that is an issue for commanders on the ground."
Shadow foreign secretary Douglas Alexander told BBC Radio 4's Today: "This was an appalling act and our thoughts should be with those who have lost loved ones as a result of this terrible incident. We should be under no illusions that it only heightens the risks facing British and international troops in the days and weeks ahead.
"That is why there needs to be a renewed diplomatic effort to try to find a negotiated solution to this conflict. … But the window of opportunity for that negotiated settlement is rapidly diminishing, which is why I welcome the fact that the Prime Minister is going to Washington. I hope that Afghanistan will be at the top of the international agenda."