At least 30 Britons are believed to have been killed in the beach massacre in Tunisia as informed sources said the eventual figure could be even higher.

At least 30 Britons are believed to have been killed in the beach massacre in Tunisia as informed sources said the eventual figure could be even higher, British newspapers reported Monday.

The official British death toll is now known to be much higher than originally reported, because of the time taken to confirm the identities of those killed. Due to the nature and location of the attack, and the numbers involved, the process has not been straightforward.

One problem is that many of those killed on the beach were wearing swimwear, and not carrying documentation that would identify them. Another problem is that, in some cases, it has taken time to find relatives able to give a visual identification. Some of the bodies have had to be identified through alternative methods, such as using dental records.

The Foreign Office will only confirm British fatalities once identification has been formally completed.

The growing number 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.

More than 600 British police have now been deployed in response to Friday’s attack in one of the biggest counter-terrorism operations since the London bombings on 7 July 2005.

Almost 400 officers were at UK airports over the weekend to speak to potential witnesses returning from the scene of the attack on a luxury hotel in the resort city of Sousse that claimed the lives of at least 38 people.

Sixteen senior Metropolitan police detectives, forensic specialists and family liaison officers arrived in Tunisia over the weekend, and more are set to join them to assist in the investigation and prevent possible further attacks.

Tunisian investigators said on Sunday that they were seeking one or more accomplices of the 23-year-old gunman, Seifeddine Rezgui, who was shot dead by police after his assault on the Imperial Marhaba hotel. They have questioned the student’s father and three of his college roommates.

Mohammed Ali Aroui, an interior ministry spokesman told the Associated Press: “We are sure that others helped, but did not participate.”

The Tunisian authorities have deployed 1,000 extra police and military to guard beaches and resort hotels, but the UK’s former counter-terrorism chief said on Sunday that extremists were plotting to kill westerners anywhere in the world, and that stopping them from accessing public areas such as beaches was near-impossible.

Meanwhile, the Queen has sent her condolences to the families of those killed and the Prime Minister David Cameron warned that Britain would not be "cowed" by the terror threat.

A gunman opened fire at tourists at a beach resort in Tunisia on Friday, leaving at least 35 people dead including foreigners, according to the first declared death toll, in what the authorities branded a "terrorist attack".