At least 15 Britons were among 39 holidaymakers killed in the Tunisian beach massacre, but the toll may well rise.
Confirming the figure, Foreign Minister Tobias Ellwood added that the Sousse atrocity was "the most significant terrorist attack on the British people" since July 7, 2005, when 52 people were killed.
Kalashnikov-wielding student Seifeddine Rezgui targeted Western tourists as he terrorised the popular resort.
One of those killed is reported to be 24-year-old fashion blogger Carly Lovett from Gainsborough, Lincolnshire.
Another British victim was named as Adrian Evans, who worked for Sandwell Council in the West Midlands as a manager in the gas services department.
Speaking in the Foreign Office, Mr Ellwood said: "Sadly, I can confirm that at least 15 British nationals were killed in yesterday's atrocity but I should stress that the number may well rise as several more have been seriously injured in this horrific attack."
He added: "This is the most significant terrorist attack on British people since 7/7 and highlights the ongoing threat of Isil."
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.
Asked if he feared further attacks on Britons abroad, he said: "We always have to be vigilant wherever we are in the world and we are doing our utmost to work with the international community to make all our places of interest safer."
He said they are "looking at the travel advice" but said they are pleased the efforts the Tunisians have put in place to increase security in the areas of concern.
Mr Ellwood said he had spent much of today on the phone with victims and families caught up in the "appalling attack".
British police have flown to the resort to help identify victims and consular teams are in hospitals and hotels looking after those affected, he said.
The July 7 attacks were the worst-ever terrorist outrage on British soil.
UK-born suicide bombers Mohammed Sidique Khan, 30, Shehzad Tanweer, 22, Hasib Hussain, 18, and Jermaine Lindsay, 19, detonated devices on three Tube trains and a bus, causing carnage.
Events are being planned to mark the tenth anniversary of the attacks.
The so-called Islamic State (also referred to as Isil) has claimed it is behind the attacks and has reportedly identified Rezgui by his jihadi pseudonym Abu Yahya al-Qayrawani.
The 23-year-old Tunisian aviation student disguised himself as a tourist and began firing at holidaymakers on a beach using the gun he had hidden in a beach umbrella.
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 other people were also wounded in the attack, including 21 who have already left hospital. Of the wounded, 25 were British.
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.
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.
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Meanwhile the Irish government said there were "grave concerns" about two more Irish citizens who were caught up in the shooting.
Foreign affairs minister Charlie Flanagan said authorities are in contact with the families of the two citizens who are still missing.
An Irish mother of two, Lorna Carty, from Robinstown, Co Meath, has already been confirmed as among the dead.
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Lincolnshire Police have appointed a family liaison officer to support Ms Lovett's family, although she is yet to be officially named as a victim by the Foreign Office.
Police officers were positioned outside her address and reporters have been asked to stay away. Her fiance was understood to be travelling back home today.