All five soldiers caught in the blast were airlifted to hospital, where one was pronounced dead.
He was not identified by name, pending notification of his next of kin. Shortly afterward a man failed to stop at a security cordon around the scene and was shot and killed, the ministry said.
A second man was also wounded.
The latest fatality brings to 61 the number of British soldiers killed in Afghanistan since the US-led drive to overthrow the Taliban government began in November 2001.