The head of the Armed Forces has insisted that Britain's military strategy in Afghanistan will not change despite the deaths of six soldiers who were killed in the deadliest single enemy attack on UK troops since the war began.

General Sir David Richards vowed the country would "hold its nerve" and continue combat operations in the region until the end of 2014 as the total number of British forces killed since the US-led invasion rose to more than 400.

The servicemen, five from the 3rd Battalion the Yorkshire Regiment and one from the 1st Battalion The Duke of Lancaster's Regiment, were on patrol on Tuesday when their Warrior Armoured Fighting Vehicle was caught in an explosion in Helmand, around 40km north of Lashkar Gah.

They are expected to be officially named by the Ministry of Defence later on Thursday after their families were informed by officials.

Prime Minister David Cameron said the news marked a "desperately sad day for our country", while Labour leader

Ed Miliband saluted "all of our fallen and those who continue to serve in the face of the gravest danger".

The Chief of the Defence staff also paid tribute to the "sacrifices" made by the Armed Forces, but warned of more deaths as Isaf allies enter the final stages of the decade-long rebuilding of Afghanistan that will see security responsibilities passed over to the Afghan Army and police.

Writing in The Times, General Richards said the progress made since entering the country in the wake of the 9/11 terrorist attacks was "truly impressive" and vowed to continue the strategy and tactics employed by British troops.

"As progress continues the work of our servicemen and women will draw down but our efforts will endure," he said.

"Sadly as we hold that course it is likely that others will lose loved ones."

General Richards added he and his troops do not "underestimate the dangers" faced in Afghanistan, but understood "the importance of the mission with which we are charged". "We will hold our nerve," he added.

Meanwhile, Defence Secretary Philip Hammond reiterated that the UK would be sticking to its military plan, stating the strategy to find an "enduring solution" in Afghanistan was "demonstrably working".

"The shocking loss of British lives this week - the worst for many years - understandably raises questions about the continued presence of UK Forces in Afghanistan: about why we are there and what we are achieving?" he said writing in The Telegraph.

"I am clear about the answers: the mission is necessary for national security. The reinvigoration of campaign strategy in the past few years is achieving our aims - building the capability of the Afghan government to maintain its own security and by extension protecting ours. Walking away is not an option."

Mr Hammond said despite this week's tragedy, the overall trend of insurgent attacks and UK casualties was "sharply" down.

General Lord Guthrie, former Chief of the Defence staff, said it was important Britain avoided a knee-jerk reaction to the deaths by changing plans and speeding up the withdrawal of troops.

Moving scenes played out at Battlesbury Barracks in Warminster, Wiltshire, the home of 3rd Battalion The Yorkshire Regiment, yesterday as the tragic news broke.

Flowers and cards of condolences were left outside the entrance, while two uniformed soldiers relit a candle next to the barracks' gates, which was put in place when around 90 soldiers from the Corunna Company were deployed less than a month ago.

A service of "quiet contemplation" in memory of the soldiers was last night held at the Minster Church of St Denys in the old market and garrison town as flags flew at half mast.

Mayor of Warminster Pip Ridout said: "It's just a devastating day for the town, there's an aura of complete disbelief and silence everywhere".

Tuesday's deaths take the number of British fatalities to 404 since the start of operations in 2001.