Two British soldiers were shot dead in southern Afghanistan as they tried to rescue a comrade, the Ministry of Defence has announced.

The troops, one from the Royal Dragoon Guards and the other from 1st Battalion Scots Guards, were killed yesterday in the Lashkar Gah district of Helmand province. They are expected to be named today.

"They died helping their friends," said Lieutenant Colonel James Carr-Smith, a spokesman for Taskforce Helmand.

"In the courageous and selfless act of attempting to evacuate an injured colleague, they themselves were shot and fatally wounded. The soldiers were part of a cordon operation providing security for a routine rotation of troops when they were killed by small arms fire," he said.

The British death toll in the Afghan campaign since 2001 now stands at 324. The latest death come as David Cameron has been forced to clarify the timetable for the withdrawal of British troops from the country.

Senior Tory backbenchers yesterday criticised the government for confusion over whether there was fixed a timetable for withdrawing troops in 2015 or was this was just a policy aim.

Cameron said he expected British troops to start leaving Afghanistan next year and complete their pullout by 2015, but only if conditions were right.

"To give people some certainty, we have said, to be clear, that in 2015 there are not going to be combat troops, or large numbers of British troops, in Afghanistan," he said.

The bodies of four British servicemen killed in separate incidents in Afghanistan will be repatriated later today. Staff Sergeant Brett Linley, 29, of 11 Explosive Ordnance Disposal Regiment, and Sergeant David Monkhouse, 35, of the Royal Dragoon Guards, both died on Saturday.

Their deaths followed those of Senior Aircraftman Kinikki Griffiths, 20, of 1 Squadron RAF Regiment, and Marine Jonathan Crookes, 26, of 40 Commando Royal Marines, who died the previous day.

The bodies of all four men will be flown back to to RAF Lyneham, Wiltshire.