At least eight Britons are among 39 holidaymakers killed in the Tunisian beach massacre.
Kalashnikov-wielding student Seifeddine Rezgui targeted Western tourists as he terrorised the popular resort of Sousse.
Tunisian authorities have confirmed the nationalities of 10 of the dead so far, with a Belgian and a German identified alongside the British victims.
Prime Minister David Cameron has warned that the public should be "prepared for the fact that many of those killed were British".
One of those killed is reported to be 24-year-old fashion blogger Carly Lovett from Gainsborough, Lincolnshire.
Miss Lovett, who had only been engaged a few months, was with her fiancee, and they were due to return home today, ITV News reported.
Friends and fellow bloggers paid emotional tributes to the former University of Lincoln student on social media.
The university's School of Film and Media tweeted: "Saddened to hear news reports of Carly Lovett in Tunisia - Graduate of our University 2013."
Some 39 people were also wounded in the attack, including 21 who have already left hospital. Of the wounded, 25 were British.
Rezgui was said to have laughed and joked as he targeted British and French tourists and sprayed them with gunfire. He was shot dead by police.
The 23-year-old Tunisian aviation student disguised himself as a tourist and began firing at holidaymakers on a beach using a gun he had hidden in a beach umbrella.
Tunisia's prime minister Habib Essid said an initial investigation found that he was not previously known to the authorities.
Scotland Yard today said police travelled to Tunisia to help investigate the killings and officers are also interviewing tourists returning at British airports.
Assistant commissioner Mark Rowley, national policing lead for counter-terrorism, said: "We've deployed a large number of officers out to Tunisia, both in terms of our own evidence gathering - forensic experts, helping the Tunisia authorities - and also we've got experts in family liaison assisting."
Mr Rowley said it was "fairly clear" the location of the Tunisian attack was chosen because of the number of Westerners present in the area, he added.
Holiday firm Tui, which runs the Thomson and First Choice brands, said that a number of those who died were its customers.
Peter Long, joint CEO of Tui Group, said he was "deeply, deeply shocked", adding: "And our whole organisation is reeling with pain to see the suffering that has taken place in Tunisia."
He said the company had so far repatriated 1,000 customers with 5,400 remaining in the area.
Travel agents' association Abta said an estimated 20,000 people were currently on holiday with its members in Tunisia but there were also others who had travelled independently.
The so-called Islamic State has claimed it is behind the attacks and has reportedly identified Rezgui by his jihadi pseudonym Abu Yahya al-Qayrawani.
The worst such attack in Tunisia's history came on the same day a man was found decapitated after an attack by suspected Islamic extremists on a French factory and a Shiite mosque in Kuwait was bombed, killing at least 25 people.
Although the attacks do not appear to be directly linked, they come after IS called for their followers "to make Ramadan a month of calamities for the non-believers".
Tension has been high in Tunisia since an attack on the National Bardo Museum in March which killed 22 people, mostly foreign tourists including a Briton.
A suicide bomber blew himself up in a failed attack on the beach in Sousse in October 2013, while 21 people lost their lives in an attack in the country earlier this year.
Mr Cameron, who today chaired a second meeting of the Government's emergency Cobra committee, said his "thoughts and prayers are with the loved ones of those killed or injured".
Speaking in Downing Street, he said: "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 added: "We are working with the Tunisian authorities to identify the final number of British casualties but I'm afraid that the British public need to be prepared for the fact that many of those killed were British."