A member of the Royal Marines has been killed during operations in southern Afghanistan on Sunday, the Ministry of Defence has said.
The UK is among the largest contributors to the Nato mission
A spokesperson said next of kin have been informed and further details will be issued after a 24-hour period.
The latest death means four members of the UK armed forces have been killed in the region in the past week.
It brings the number of UK military fatalities in Afghanistan since operations began in 2001 to 68.
The UK is among the largest contributors to the Nato mission in Afghanistan, with 7,100 troops based in Helmand province in the south of the country.
Up to 1,500 UK troops and 500 Afghan, Estonian, Danish and US soldiers have been involved in a mission to try to force Taleban insurgents further up the Gereshk Valley.
Lance Corporal Alex Hawkins, 22, of the Royal Anglian Regiment died on Wednesday, when a roadside bomb struck the vehicle he was travelling in.
Guardsman David Atherton, 25, from the Grenadier Guards, died after being shot during a firefight on Thursday after firing an anti-tank missile at Taleban positions.
And Sergeant Barry Keen, 34, serving with 14 Signal Regiment, was fatally wounded when a single mortar round landed next to him on Friday.