At least 38 people were killed when a Tunisian student disguised as a holidaymaker opened fire at a resort in Port el Kantaoui.
Britain said Saturday that at least 15 of its citizens were killed in the attack on a Tunisian beach resort, its worst loss of life in a terror incident since the 2005 London bombings.
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 (90 miles) 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.
Prime Minister David Cameron had earlier warned that Britain needed to prepare "for the fact that many of those killed in the attack were British".
He condemned those responsible as "evil", saying the victims were "innocent holidaymakers relaxing and enjoying time with their friends and families".
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.
British police have sent forensic experts and detectives to Tunisia to help identify victims and gather evidence.
Officers were also interviewing many of the holidaymakers who flew home on Saturday, 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.
The TUI group, which includes Thomson and First Choice, sent ten planes on Saturday to repatriate 1,000 customers and hopes to return a total of 2,500 by late Sunday.
Jet2, which has more than 1,000 customers on holiday in Tunisia, said it had repatriated 205 people and would send two more planes to collect more over the weekend.
Both companies cancelled their holidays to Tunisia for the coming week.
'They're inside, run'Among the British dead were Carly Lovett, a 24-year-old fashion blogger, Adrian Evans, a local authority worker, and his nephew, student Joel Richards, media reports said.
Meanwhile harrowing stories of the massacre continued to emerge from survivors.
Olivia Leathley, from Manchester in northwest England, described how she and her boyfriend Mike Jones heard grenades and gunfire and saw "hundreds of people running and screaming from the beach".
"Somebody shouted, 'they're inside, run!'. We just ran as far away from the bullets as we could. It was all happening so quickly, it was deafening," she told the BBC.
The couple found shelter in a small security lodge, and later discovered that the gunman was killed "about 400 yards" from where they were hiding.
Security stepped upThe shooting was the second attack on tourists in Tunisia within three months, and came the same day as a bombing in Kuwait, which was also claimed by the IS group, and a suspected Islamist murder at a factory in France.
Cameron on Saturday chaired a meeting of the government's Cobra emergency committee, and has also spoken to the presidents of Tunisia and France and the emir of Kuwait.
"These savage terrorist attacks in Tunisia, Kuwait and France are a brutal and tragic reminder of the threat faced around the world from these evil terrorists," he said.
He added on Twitter: "Together, we'll make sure terrorists do not win."
Britain's terrorism threat level is currently at severe, the second highest of five levels, meaning an attack is highly likely.
Security was stepped up at major public events on Saturday, including armed forces day celebrations and London's gay Pride march.