Announcing the toll, Foreign Office minister Tobias Ellwood warned the numbers killed in Friday's shooting "may well rise, as several more have been seriously injured in this horrific attack".
At least 38 people were killed when a Tunisian student disguised as a holidaymaker opened fire at a resort in Port el Kantaoui near Sousse, about 140 kilometres south of Tunis.
Another 39 people were injured, including 25 Britons, in the attack which was claimed by the Islamic State group, which controls large parts of Iraq and Syria.
He condemned those responsible as "evil", saying the victims were "innocent holidaymakers relaxing and enjoying time with their friends and families".
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The attack represents the largest British loss of life in a terror incident since four suicide bombers attacked the London transport system on July 7, 2005, killing 52 people.
"This is the most significant terrorist attack on British people since 7/7 and highlights the ongoing threat of ISIL," said Ellwood, who has personal experience of terrorism, after his brother died in the 2002 Bali bombing.
Officers were also interviewing many of the holidaymakers who flew home today, in particular looking for any phone footage taken of the incident.
About 20,000 British tourists were on package holidays in Tunisia at the time of the attack, according to ABTA, the country's largest travel association.
Several travel firms laid on special flights to repatriate survivors and other tourists desperate to get home.