Corporal Channing Day, 25, from 3 Medical Regiment, became the third British woman soldier to die in the country since the conflict began more than a decade ago.

One of her two sisters, Laken, said last night: "I am the proudest sister ever, her legacy will live on."

Cpl Day, from Comber, near Belfast, was killed with Cpl David O'Connor, 27, a Royal Marine from 40 Commando, after her unit shot an Afghan policeman by mistake, according to local police.

The policeman, who was not wearing his uniform and was carrying a gun, went to wash his hands in preparation for prayers about 50 yards from his checkpoint. Cpl Day's unit mistook him for a Taliban insurgent and opened fire, local officials said.

Farid Ahmad Farhang, a spokesman for the provincial police, said another British unit on patrol nearby assumed it was under attack and fired back, killing Cpl Channing and the Royal Marine. The circumstances surrounding the deaths remained confused last night, however. Other reports claimed that the dead policeman's colleagues returned fire, while British officers were also investigating the possibility that the patrol was deliberately attacked by the policemen.

On Thursday night all the Ministry of Defence would say about the incident was that the soldiers died as a result of "small arms fire". The deaths followed a series of "green on blue" attacks, in which Afghan police or soldiers have turned their weapons on their Nato partners.

Cpl Channing had been determined to join the Army from her first day at secondary school.

Paul Maxwell, acting principal at Strangford Integrated College in Carrodore, said she received a "glowing report" after she did a week's work experience with the Forces. She also represented Northern Ireland Ladies Football at junior level.

Mr Maxwell added: "She came into school and said, 'I want to join the Army'. She was an excellent sportsman, a very active, outgoing girl. It is hard to believe that someone so young can be killed in those circumstances." A friend added: "She was so proud to be in the Army, so proud to wear the uniform.” Cpl Day achieved top grades in PE, excelling at gymnastics, trampolining and football. Mr Maxwell said her sporting prowess revealed a gritty determination to succeed.

“It was not just that she was good but that she always showed commitment, she stayed after school and did all the practice,” he added.

She started at Strangford College in 1998, the year after it was established, and left in 2003. The integrated education movement in Northern Ireland was set up against a prevailing system which encouraged children to attend mainly Protestant state-controlled schools or mainly Catholic schools run by the Church, a legacy which some have criticised for entrenching sectarianism from a young age. Mr Maxwell said: “She came in the second year of the school’s existence and at that stage the school had no funding, none until 2000. It was a leap of faith to send your child to an integrated school of that nature.”

The school is in a Protestant area, where service in the Army would be more acceptable. Cpl Day was of mixed race but Mr Maxwell said this did not affect how she was perceived at school or by her friends.

“Because of her sporting skills she was able to fit right in,” he said. “She came to a school that could have closed because there was no funding, it was a risk for mum and dad to send her.”

Cpl David O’Connor from Havant, Hants, joined the Royal Marines in 2002 and was hailed by his superiors as “great leader, great companion and great friend”.

Lt Col Matt Jackson, Commanding officer of 40 Commando, said: “He was one of the best. He was loved and respected by those around him and was relentless in the pursuit of excellence in his duties.

“Utterly professional, his four tours of Iraq and Afghanistan marked him as one of the most devoted individuals in the Commando. Brave, committed and humorous it is difficult to find words which can possibly fill the void which had been left by this truly excellent junior commander.” His parents, Rosemary and George, and brother Phil, said: “David’s family and friends are greatly saddened by his loss.”