At least 30 British people are believed to have died in the Tunisia terror attack, according to Sky sources.
If confirmed it would double the number of Britons known to have been killed in the deadly rampage by the gunman Seifeddine Rezgui.
It would also mean UK citizens make up the majority of the 38 victims of the atrocity at the beach resort in Sousse on Friday.
The massacre is the worst loss of life of Britons in a terror attack since the 7/7 bombings.
The growing list of UK fatalities comes as footage emerged of 24-year-old Rezgui sprinting along the shore with his Kalashnikov rifle after opening fire on tourists.
On Saturday, Sky News obtained pictures of the armed attacker strolling along the beach.
Witnesses have described how he was “very, very calm” when he opened fire on holidaymakers after emerging from the sea where he had apparently washed himself.
He was also said to be grinning and ignoring pleas to stop the attack.
The slaughter only ended when he was shot dead by police. A bomb was found on his body.
His father Hakim Rezgui reportedly said he was “shocked” by what happened and did not know who “put these ideas in his (son’s) head” and “warped his mind”.
He said he was “really sorry”, was “so ashamed” and felt like he had died along with the victims.
Mr Rezgui and three of his son’s roommates at university have been detained and were being questioned in the capital Tunis.
Investigators are searching for one or more accomplices in the attack, according to one official.
A phone found by a swimmer reportedly showed the gunman called his father just before the assault and authorities believe the suspected accomplices provided the Kalashnikov assault rifle to Rezgui.
Thousands of British tourists have been returning to the UK after cutting short their holidays on the Mediterranean resort.
Witness Lee Inwood said his wife was “confronted” by the terrorist on the beach before the attack.
The couple were walking towards the sea when she stumbled on a children’s spade and nearly fell into a group of people, Mr Inwood said.
The Briton said among the four local men and four local women was the gunman who confronted her.
Mr Inwood said the group was later acting “very strangely” and the women were taking photos of the men with tourists’ children and also pictures of holidaymakers on sunloungers without permission.
He said “something was not right, it feels weird” and claimed local people do not normally “hang around” that area and leave the tourists alone.
He thought the photos were “trophy pictures”.
Mr Inwood also said he was told by his hotel director that there were three gunmen in the attack, not just one that authorities claimed. He said he had given a statement to British police.
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.”
source: Sky News