Five British citizens have been killed in a gun massacre in Tunisia on Friday, Foreign Secretary Philip Hammond said, warning that the toll could rise.
Hammond said that a "high proportion" of the casualties of the attack were expected to be British.
Some 37 people have been killed and a further 36 wounded, according to the authorities in Tunis.
"It's clear that there have been a number of people killed," he said outside the Foreign Office in London, after chairing a government emergency response meeting.
"Because of the nature of the composition of the tourist population in this part of Tunisia we have to assume that a high proportion of those killed and injured will have been British.
"We have had reports from families of those involved in the incident that allow us to confirm that at least five Britons have been killed in this incident.
"But I should warn that we must expect that there will be more reports of fatalities as we establish the detail on the ground," he said.
Hammond said Britain had a consular team on the ground in Sousse, the beach resort where the attack occurred, with two further "rapid response teams" arriving overnight Friday.
He said he had held a conference call with tour operators and there would be a further government emergency response meeting on Saturday morning.