A renegade Afghan soldier killed three British troops during a joint patrol today in southern Helmand province, a provincial security source confirmed.
Two more British soldiers were wounded in the attack which happened near Lashkar Gah, the provincial capital of the restive province, where some 9,000 British troops have been struggling to quell Taliban insurgents.
Nato and US forces commander General David Petraeus said a joint investigation was underway into the incident, which killed three Nato soldiers. He pleaded for continued unity between 150,000 international troops in the country and local security forces.
"We have sacrificed greatly together, and we must ensure that the trust between our forces remains solid in order to defeat our common enemies," Gen Petraeus said in a statement. "This is a combined, joint mission, Afghan and Alliance troopers fighting shoulder-to-shoulder against the Taliban and other extremists."
Today's attack is not the first time foreign troops have been killed by renegade Afghan security forces, raising concern among some in the West about the degree of Taliban infiltration in state security forces trained and funded by Nato as part of its fight against the resurgent militants.
"We heard about this this morning with regret and the president was upset to hear this," said Waheed Omer, spokesman for Afghan president Hamid Karzai.
In the deadliest such attack yet, an Afghan policeman killed five British soldiers in a training base in southern Helmand province last November.
A month later, an Afghan soldier shot dead a US service member and wounded two Italian soldiers in a joint Nato and Afghan base in northwestern Badghis.
There have been several other attacks by men in army and police uniforms against government and international forces.