The British death toll in the Tunisian beach massacre is expected to double to at least 30, sources have said.
The latest figure comes after Foreign Secretary Philip Hammond warned that it was highly likely a "significant number" of victims yet to be identified would be British.
David Cameron has pledged the UK will not be cowed by the atrocity, vowing to "stand up for our way of life" and insisting the "great British spirit" would triumph in the face of adversity.
A total of 38 people were killed when a gunman opened fire on a beach in the Sousse resort on Friday, with the the Foreign Office already confirming 15 of them were from Britain.
At a briefing following an emergency meeting of cobra yesterday, Home Secretary Theresa May also said the British deaths figure was expected to rise as more information comes out.
Three Irish people are also among the dead.
Mr Hammond said there had been delays in identifying victims because many were "dressed for the beach, not carrying ID physically on them".
He added: "There are a significant number of victims who have not been positively identified at this time and it is highly likely that a significant proportion of them will be British."
He said it was "extraordinarily difficult" to predict where the next attack will happen and was no more likely to be in Tunisia than in a European city.
"Our agencies have been very frank about this over a long period of time now, they cannot guarantee that we will be safe from this kind of self-radicalising lone-wolf attack.
"It is the most difficult type of attack to detect and predict and therefore the most difficult kind to protect against."
The Queen sent her condolences to the families of those killed in the atrocity.
She said the incident had left her and the Duke of Edinburgh "shocked".
The monarch also sent her "deepest sympathy" to those injured in Friday's brutal slaughter.
As tributes were paid to the victims by family and friends, Buckingham Palace released a statement from the Queen saying: "Prince Philip and I were shocked to learn of the attack on British tourists in Tunisia on Friday.
"We send our sincere condolences to the families of those who were killed and our deepest sympathy to the people who are still fighting for their lives in hospital, and those who have been seriously injured.
"Our thoughts and prayers are with those of all countries who have been affected by this terrible event."
The killing spree by Kalashnikov-wielding student Seifeddine Rezgui targeted western tourists on the beach at the RIU Imperial Marhaba and the RIU Bellevue and only ended when he was shot dead by police. A bomb was found on his body.
Investigators have revealed they are looking for at least one more accomplice, with an interior ministry spokesman telling the Associated Press they are sure that Rezgui had help.
Victims of the atrocity include three generations of the same Midlands family. Patrick Evans, believed to be 78, his son Adrian, and his 19-year-old grandson Joel Richards. Joel's brother Owen, 16, survived the attack.
Tunisia terror attack: Father of gunman says he is 'ashamed' of his son's actions
Others included Lisa Burbidge, from Whickham, Gateshead, and Trudy Jones, of Blackwood in Gwent, south Wales, who was named as among the dead by her MP, Chris Evans.
Fashion blogger Carly Lovett, 24, from Gainsborough, Lincolnshire and grandfather Bruce Wilkinson, 72, believed to have been a retired power station worker, from Goole, East Yorkshire, were also killed.
Engineer Stephen Mellor from Bodmin in Cornwall was also killed as he shielded his wife Cheryl on the beach, she told the Mirror.
The Irish victims were identified as mother-of-two, Lorna Carty, from Robinstown, Co Meath, and married couple Laurence and Martina Hayes, both aged in their 50s, from the town of Athlone in Co Westmeath.
Thousands of British tourists have been returning to the UK after cutting short their holidays on the Mediterranean resort
Scotland Yard said more than 600 officers were involved in what was its largest counter-terrorism operation since the 7/7 bombings.
Assistant Commissioner Mark Rowley, national policing lead for counter-terrorism, said 16 detectives and forensic experts were already in Tunisia to help local officers, with almost 400 meeting survivors at UK airports to identify possible witnesses.
He said: "Because of the scale of the attack, the numbers of fatalities and the international nature of it, it is likely that several hundred counter-terrorism and other police officers and staff will be working on this case for some time."
Downing Street has confirmed that Mr Cameron will be chairing another meeting of the Cobra emergency committee this morning.
Minister for Foreign Affairs Charlie Flanagan has said that he currently has "no reason to believe" of any further injuries to Irish citizens in the wake of the Tunisia beach massacre.