At least five Britons are among the dead after an attack on a beach near the popular Tunisian resort town of Sousse, UK Foreign Secretary Philip Hammond has said.
Tunisia’s health ministry has said at least 37 people are dead, mostly foreigners.
The UK death toll could rise, Mr Hammond said, with a “high proportion” of casualties expected to be British.
Tunisian officials say one gunman was shot dead and another is being pursued.
They said one attacker, who had posed as a swimmer but was carrying a rifle under a parasol, was shot dead in an exchange of fire with police.
Local media reported that a second attacker had been arrested, but this has not been confirmed.
Tunisians, Germans, Belgians and at least one Irish citizen are among the dead. A further 36 people are injured, the health ministry said.
Tourists described “bullets whizzing around us” and scenes of “panic” at the Port El Kantaoui district.
