The British Foreign Office said at least 15 Britons were among 39 holidaymakers killed in "the most significant terrorist attack on the British people" since the 2005 7/7 attacks.
Speaking in the Foreign Office, Foreign Minister Tobias Ellwood said this number "may well rise" as several more people have been seriously injured in the Tunisia hotel massacre.
Mr Ellwood said this act of "evil and brutality" demonstrates why this kind of extremism has to be confronted "wherever this happens" at home or abroad.
Tunisian authorities have confirmed the nationalities of 10 of the dead so far, with a Belgian and a German identified alongside the British deaths.
Photographer Carly Lovett was the first Briton to be identified as a victim of the massacre. The University of Lincolnshire graduate was on holiday with her boyfriend at the time, who is due to fly back to the UK on Saturday.
Relatives and friends of British people in Tunisia were appealing for information and help locating their loved ones on social media as the wait for victims to be be officially named continued.
Shocking stories from survivors emerged overnight and throughout Saturday. Travel operators sent flights to repatriate thousands of Britons on holiday in the country, with the first wave arriving back in the UK this morning.
The gunman, who was cornered and shot dead by police, has been identified as Seifeddine Rezgui, a young student who was not previously known to Tunisia’s security services.
Additional reporting by agencies
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