This is the first time a female British soldier has died in action in the troubled country and takes the UK's death toll over a bloody 10-day period to nine.

The troops were taking part in a planned operation east of Lashkar Gah in Helmand Province on Tuesday afternoon when their vehicle was caught in an explosion.

Three of the soldiers were killed in the blast and the fourth was pronounced dead on arrival at Camp Bastion. A fifth soldier wounded in the attack is said to be in a stable condition.

It is understood that the woman was serving with the Army's Intelligence Corps and the men were special forces reservists with the 23rd SAS Regiment.

The Ministry of Defence would not confirm what kind of vehicle the soldiers were in when they were hit by the blast, but it was reported to be a Snatch Land Rover.

Coroners and opposition politicians have voiced concerns that troops in Afghanistan have not been given enough protection against roadside bombs. Experts have called for military vehicles to be given extra protection against improvised explosive devices.

Prime Minister Gordon Brown expressed his "deepest condolences" to the families of the four soldiers killed. He said UK forces had made "great progress" against the Taliban, noting that the insurgents were increasingly using mines and roadside bombs rather than open combat.

A total of 106 British service personnel have died since the start of operations in Afghanistan in November 2001. The recent series of deaths marks a sharp upturn in violence against troops after a period of relative stability.

The incident in Helmand was the biggest single loss of life for British troops since September 2006, when 14 personnel were killed when an RAF Nimrod came down near Kandahar.