At least 15 Britons were among 38 holidaymakers killed in the Tunisian beach massacre as names of those believed to be victims continue to emerge.
The Sousse atrocity is "the most significant terrorist attack on British people" since July 7, 2005 when 52 people were killed, Foreign Minister Tobias Ellwood said.
He confirmed that at least 15 British nationals were killed in the attack but said that number "may well rise as several more have been seriously injured".
Mr Ellwood 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.
His killing spree on the beach at the RIU Imperial Marhaba and the RIU Bellevue ended when he was shot dead by police. A bomb was found on his body.
Home Secretary Theresa May will chair a meeting of the Government's emergency cobra committee this morning.
The Foreign Office has updated its travel advice to warn that further terrorist attacks in Tunisia are possible and urged people to be vigilant.
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: "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 and Birmingham County FA said he was an active member of its youth council.
In a statement, it said: "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 Connolly's GFC also paid tribute to Mr Richards, tweeting that the club was "devastated" to learn of his death.
They said he was an "exceptionally-talented footballer" who represented both club and county "with conviction" on numerous occasions.
Mr Richards's 16-year-old brother Owen is reported to have survived the attack.
A man who thought he may have seen his mother on TV as she was being taken away on a stretcher later tweeted that he believed she had died.
Conor Fulford, from Tamworth, Staffordshire, tweeted: "Hi Everyone, Sorry to tell you we lost my mother Sue Davey tonight but i want to thank everybody that tried to help me & my family ..."
He said his mother was a "strong" woman, adding: "Love you always Mom, I've got my teddy bear you got me tonight, Rest easy xxxx".
She was on holiday with her partner Scott Chalkley. His son Ross Nayler, from Derby, had launched an online plea for any information on his father but last night posted an update online saying Mr Chalkley was "no longer with us".
He wrote: "Tonight me and Brad lost our Dad, unfortunately he was caught up in the sickening attacks in Tunisia and is no longer with us."
He added: "I'm sure many of you have seen the news about Sue as well so all I ask is that people let both family's grieve in peace, we are all trying to come to grips and process with what's happened." (sic)
Another of those killed is reported to be 24-year-old fashion blogger Carly Lovett from Gainsborough, Lincolnshire.
Lincolnshire Police appointed a family liaison officer to support Ms Lovett's family and police officers were positioned outside her address.
Her fiance was understood to be travelling back home.
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."
Asked if he feared further attacks on Britons abroad, Mr Ellwood 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."
Meanwhile, 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.
An Irish mother-of-two, Lorna Carty, from Robinstown, Co Meath, has already been confirmed as among the dead.
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.
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.
British police have flown to the beach resort to help identify victims and consular teams are in hospitals and hotels looking after those affected.
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".
People gathered in Sousse last night for a protest against terrorism.
Crowds gathered after dark with some holding candles while others carried flags including those of the United Kingdom and Tunisia.
Some people held posters aloft proclaiming "Peace" and "Sousse will never die".