Afghan forces accidentally fired on British troops in Helmand Province at least 19 times over a three-and-a-half year period, military incident logs reveal.
Four of the "friendly fire" incidents resulted in casualties, although extracts from official files released by the Ministry of Defence do not record any UK personnel being killed or seriously injured.
There were also at least 10 cases between January 2008 and June this year where British forces mistakenly fired at Afghan soldiers, police and security service officials, resulting in seven deaths.
In the worst incident, in the Lashkar Gah district of Helmand in October 2008, three Afghan National Police (ANP) officers were killed and one very seriously injured when UK troops opened fire on them.
The log notes: "Acting on a specific threat warning and in the belief that no Afghan or Isaf (Nato) forces were in the area, UK forces fired at individuals assessed to be preparing an attack. The individuals were subsequently identified as ANP officers preparing an ambush against insurgent forces."
In another incident a British night patrol in Sangin in northern Helmand in August last year fired on a motorcycle that failed to stop despite warnings. One of the riders was shot in the head and died, and the other was injured. It later turned out they were both ANP officers.
UK troops also killed a member of Afghan intelligence in Sangin in September 2008, an Afghan soldier in Nad-e-Ali in October 2008 and an Afghan border police officer in Garmsir in April 2009.
Not all the incidents ended in tragedy. A joint British and US patrol came under attack from an Afghan National Army (ANA) compound near Camp Bastion, the main UK base in Helmand in July 2009. The log observes: "When the ANA forces realised they had been firing on friendly forces they invited the patrol in for tea."
Details of the friendly fire incidents were released by the military's Permanent Joint Headquarters (PJHQ) based at Northwood in Middlesex in response to a Freedom of Information request.
A Ministry of Defence spokesman said: "Any friendly fire incident is one too many and thankfully such events are isolated and extremely rare - particularly as the vast majority of UK and Afghan forces operate alongside each other on a daily basis and in many cases live and work together."