The Queen has sent her "sincere condolences" to the families of those killed in the Tunisian beach massacre.
The monarch said she and the Duke of Edinburgh were "shocked" by Friday's gun attack and she also offered their "deepest sympathy" to the injured.
Officials fear the UK death toll is likely to be in the mid to high 20s, out of 38 victims in total. Three of the victims were three generations of the same family.
Relatives of others still missing are anxiously awaiting any news of their loved ones.
Daniel Welch, who has not heard from his grandfather John since the attack, said "we can only expect the worst".
The Foreign Office has updated its travel advice to warn that further terrorist attacks in Tunisia are possible, and urged people to be vigilant.
Scotland Yard says its investigation into the Tunisian attack is "likely to be one of the largest counter-terrorism deployments" since the London 7/7 bombings in 2005.
The Met said the operation currently involves more than 600 officers and staff, with many sent to British airports to speak to returning holidaymakers. It has already deployed 16 officers to Tunisia, including forensic specialists and family liaison officers.
Speaking after she chaired a meeting of the government's emergency Cobra committee, Home Secretary Theresa May said Foreign Office staff were working round the clock in Tunisia and a 24-hour crisis centre was up and running in the UK.
It is the biggest terrorist attack on British citizens since the 7/7 bombings, the tenth anniversary of which is Tuesday next week.

Downing Street's flags are flying at half-mast in memory of the British victimsThe British victims have not been officially identified but the names of some have been confirmed by friends and family:
Carly Lovett, 24, a beauty blogger and photographer from Gainsborough in Lincolnshire who was first to be named
Sue Davey and her partner
Scott Chalkley, both in their 40s, whose deaths were confirmed by their respective sons
Adrian Evans, his father, 78-year-old
Patrick Evans and nephew
Joel Richards, 19, from Wednesbury.
Bruce Wilkinson, 72, from Goole in Yorkshire.
Lisa Burbidge, in her 60s, from Whickham, Gateshead
Claire Windass, 54, from Hull, was on the beach with her husband, who survived
Prayers have been said for
Jim and
Ann McQuire, from Cumbernauld
Trudy Jones, 52, from Blackwood, Caerphilly county.
Three victims from the same family
One British victim was named as Adrian Evans, who worked for Sandwell Council as a manager in the gas services department.
A council source described the news as "horrendous" and said their thoughts were with the family and council colleagues.
The third victim was later named as his nephew, 19-year-old local football referee Joel Richards.
Mr Richards, who played for James Connollys GFC in Warwickshire, was described by friends on social media as "ambitious" and "one of a kind".
Connollys GFC released the following statement last night: "James Connollys GFC is devastated to learn of the death of one of our players, Joel Richards, during the horrific attack in Tunisia."
Patrick Evans, 78, who was Mr Richards' grandfather and Mr Evans' father, was also killed.
Carly Lovett, Adrian Evans and Mr Evans' nephew Joel Richards, left to right
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