Six soldiers have been killed in the single worst enemy attack on British troops of the campaign in Afghanistan .

The servicemen were on a security patrol yesterday when their Warrior Armoured Fighting Vehicle was caught in an explosion on the border of Kandahar and Helmand provinces.

The deaths take the total number of British forces killed on operation to more than 400 since the US-led invasion in the wake of the 9/11 terrorist attacks.

Prime Minister David Cameron said the news marked a "desperately sad day for our country".

The servicemen, five from the 3rd Battalion the Yorkshire Regiment and one from the 1st Battalion The Duke of Lancaster's Regiment, were struck around 40km north of Lashkar Gah.

Next of kin have been informed.

Mr Cameron said: "This is a desperately sad day for our country and desperately sad of course for the families concerned.

"It is a reminder of the huge price that we are paying for the work we are doing in Afghanistan and the sacrifice that our troops have made and continue to make."

Labour leader Ed Miliband echoed his remarks, adding: "This is a dark day. We salute all of our fallen and those who continue to serve in the face of the gravest danger."

Moving scenes played out at Battlesbury Barracks in Warminster, Wiltshire, the home of 3rd Battalion The Yorkshire Regiment, where the dark green battalion flag emblazoned with a gold lion and white rose flew at half-mast.

Two uniformed soldiers relit a candle next to the barracks' gates, put in place when around 90 soldiers from the Corunna Company were deployed less than a month ago.

Beneath it, text read: "This flame serves to remind us of the commitment that the soldiers of this battalion are currently making on operations. It will be extinguished when the last soldier of the battalion returns safely to this base."

A woman and a young boy quietly laid some pale yellow roses under the sign marking the barracks' entrance.

A card tucked inside the floral tribute read: "Dear 3 Yorks, Tragic news. Warminster is proud and will always consider you 'our boys'."

Former serviceman Lee Thomas, 43, who lives in the town, spoke of his sadness as he arrived to lay a bunch of red and yellow flowers in tribute to the "young lads".

He said: "It's just a shock. Any loss is tragic. I've come here because of the respect these boys have earned and deserve."

Mr Thomas, who worked for the RAF tactical support wing, added: "These lads deserve a lot more than what they are getting. They don't get enough support from the Government and what are the Government doing? They are sacking them left, right and centre."

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