Five British soldiers have been shot dead by an Afghan policeman at a checkpoint in an incident which is now under investigation, officials said on Wednesday.

"The soldiers were all killed as a result of gunshot wounds sustained in an attack in the Nad-e?Ali district of Helmand Province during the afternoon of 3 November 2009," it said in a statement.

"The attack is subject to investigation."

Military spokesman Lieutenant Colonel David Wakefield said on BBC radio that the soldiers killed had been mentoring Afghan police and living inside the checkpoint.

"It would appear... that an individual Afghan national policeman, possibly acting in conjunction with another, started firing within the checkpoint," he said.

The incident brings to 229 the number of British troops who have been killed in Afghanistan.

It makes this the bloodiest year for the armed forces since the Falklands War in 1982.

Three of the soldiers were from the Grenadier Guards and two from the Royal Military Police. Families have been informed.