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 British people" since July 7, 2005, when 52 people were killed.
Speaking at 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."
Kalashnikov-wielding student Seifeddine Rezgui targeted Western tourists as he terrorised the popular resort.
One of the British victims was named as Adrian Evans, who worked for Sandwell Council in the West Midlands as a manager in the gas services department.
The council leader Darren Cooper said: "We are very sad to hear that one of our employees has been killed in the terrorist attack in Tunisia.
"Adrian Evans was a very popular and long-serving employee, who was manager of our Gas Services Team.
"We are absolutely devastated for Adrian's family, friends and colleagues and our thoughts are with them and all those affected by this tragedy."
Joel Richards, reported to be Mr Evans's nephew, was also named as a victim.
A statement from Birmingham County FA said: "Joel was an active member of the Birmingham County FA Youth Council.
"A young, talented referee with the world at his feet, he was highly thought of and will be sadly missed. Our thoughts and prayers are with his family and friends at this tragic time."
He is thought to have been a student at the University of Worcester.
Birmingham-based Gaelic football club James Connollys GFC also paid tribute to Mr Richards, tweeting: "James Connollys GFC is devastated to learn of the death of one of our players, Joel Richards, during the horrific attack in Tunisia.
"Joel was an exceptionally talented footballer, who represented both club and county with conviction on numerous occasions.
"Details of his funeral arrangements will follow; our thoughts are with his family and friends at this tragic time."
An Irish mother of two, Lorna Carty, from Robinstown, Co Meath, has already been confirmed as among the dead.
Another of those killed is reported to be 24-year-old fashion blogger Carly Lovett from Gainsborough, Lincolnshire.
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.
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."
British police have flown to the beach resort to help identify victims and consular teams are in hospitals and hotels looking after those affected.
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 with 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".
The July 7 attacks were the worst-ever terrorist outrage on British soil.
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.
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.
While Assistant Commissioner Mark Rowley, national policing lead for counter-terrorism added that 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.
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.
A member of Norfolk Police staff and his wife were injured by the gunfire.
The force said Tony Callaghan, who works at North Walsham, and his wife, Christine, needed hospital treatment but their injuries are not life-threatening.
He was shot in the leg while his wife had her femur shattered.
They were staying at the Imperial Hotel and were near the pool when they heard the attack begin.
Mr Callaghan, who spent 23 years in the Royal Air Force, told the Associated Press: "I knew it was gunfire... The hotel was being attacked."
He said they then joined 40 other people in taking refuge in the administrative offices of the hotel.
They made their way to the first floor "but then we were trapped", he recalled.
Mr Callaghan, 63, told people to hide because the gunman was following "and shooting coming up the stairs".
His wife, 62, stumbled in the corridor and "was screaming 'Help me! Help me!'", her husband said shortly before going for surgery.
Home Secretary Theresa May will chair a meeting of the Government's emergency cobra committee tomorrow morning.