Six British soldiers were missing and believed to have been killed after their armoured vehicle was hit by an explosion while they were on patrol in the Helmand area of southern Afghanistan, the defence ministry said on Wednesday.

If the soldiers are dead, the incident will be the biggest single loss of life for British troops in Afghanistan since 2006 and take the overall death tally for British forces there to 404 since the 2001 U.S.-led invasion to oust the Taliban.

The soldiers, five from the 3rd Battalion the Yorkshire Regiment and one from the 1st Battalion The Duke of Lancaster's Regiment, were on a mounted patrol when their Warrior Armoured Fighting Vehicle was struck, the ministry said.

I utterly condemn those responsible for this incident who will ultimately fail to derail a mission that is protecting our national security at home and making real progress in Helmand Province, Defence Secretary Philip Hammond said in a statement.

It is because of the continuing efforts of our armed forces ... that we are on course to build an Afghanistan that can stand on its own two feet when UK combat operations cease at the end of 2014.

Britain has some 9,500 soldiers in Afghanistan, a number due to be reduced in phases as it ends combat operations in the next two years.

(Reporting by Michael Holden; Editing by Ben Harding)