8 British soldiers killed in Afghanistan
British military deaths in Afghanistan surpass Iraq toll
The Ministry of Defence said five soldiers were killed by an explosive device while on patrol in Helmand province.
Three other combat deaths were announced earlier in the day, making it one of the worst days for British forces since the Afghan war began seven years ago.
The names of the soldiers have not been released, but next of kin have been notified, the ministry said.
Friday's fresh casualties mean 15 British soldiers have been killed in the last 10 days as fighting in Helmand Province intensifies.
Speaking at the G8 summit in L'Aquila, Italy, British Prime Minister Gordon Brown defended the Afghan mission, his voice faltering as he expressed sympathy to the families of those who have been killed in recent days.
"Our resolution to complete the work we have started is undiminished," he said. "It is in tribute to the members of our forces who have given their lives that we should succeed in the efforts we have begun."
But Brown also warned it has been "a very hard summer, and it is not over."
The latest casualties take Britain's death toll in Afghanistan to 184. A total of 179 British military personnel have been killed in Iraq since the 2003 U.S.-led invasion.