The spread of Islamic State (IS) into Libya helped sow the seeds for the Tunisia beach attack, the Foreign Secretary has said.
Philip Hammond's comments came as it was revealed that the gunman responsible for the massacre attended a terrorist training camp in Libya at the same time as attackers who killed 22 people at the Bardo Museum in capital Tunis in March.
Mr Hammond said: "I think the thing that has changed is the spread of Isil (IS) into the ungoverned territory of Libya, a neighbouring country to Tunisia."
Seifeddine Rezgui, a 23-year-old student, shot dead 38 people on the beach in the resort of Sousse on Friday.
Authorities in Tunisia are quizzing seven suspected associates of Rezgui.
They have said he acted alone during the rampage but had accomplices who supported him before, providing him with weapons and logistical support.
Tunisian president Beji Caid Essebsi said an investigation was under way into security failures and there would be armed tourist police on beaches.
The president said heightened security had been planned from July 1 to coincide with the Muslim holy month of Ramadan, but that attacks on sunbathing tourists had not been anticipated.
Mr Essebsi told Europe 1 radio: "It is not a perfect system - it is true we were surprised by this affair. They took measures for the month of Ramadan but never did they think the attack would be on the beaches against tourists, and the system of protection was set to start July 1."
Tunisian secretary of state for the interior ministry Rafik Chelli disclosed that it had been established that Rezgui visited a jihadi training camp in Sabratha, western Libya, at the same time as two terrorists who killed 22 at the Bardo Museum in capital Tunis in March.
He crossed the border "secretly" and passed into Libya in January, Mr Chelli said.
"It has been confirmed that the attacker trained in Libya with weapons at the same period as the Bardo attackers," said Mr Chelli. "He crossed the borders secretly."
Mr Cameron spoke yesterday with Tunisia's ambassador in London about what assistance Britain can give the north African country to improve security and strengthen its democratic system.
The Government is "working closely" with the families of those killed in Sousse, and has offered to arrange for the bodies of their loved ones to be flown by RAF plane to Brize Norton in a "very dignified and respectful" way before being transported on to their home areas, Downing Street said.
A single inquest covering all the British victims is to be opened by the West London coroner.
The family of Janet and John Stocker have confirmed "with regret and great sadness" that the "happiest, most loving" couple died in the Tunisia shootings.
John Stollery, 58, a social worker from Nottinghamshire, was on holiday with his wife Cheryl and son when he was killed. Retired scientist David Thompson, from Tadley, Hampshire, is also believed to have been killed.
All wounded Britons have been brought back to the UK, with four severely injured holidaymakers flown home in an RAF C17 transport plane accompanied by "medevac" teams. They are being treated at hospitals in Birmingham, Oxford, Plymouth and London.
Mr Cameron has announced that a minute's silence will be held in memory of the victims at noon on Friday, a week after the outrage. Flags are expected to be flown at half-mast over Government departments and Buckingham Palace that day.
A key strand of the Government's strategy to counter extremism is coming into force tomorrow. New legislation passed earlier this year places a statutory duty on bodies including prisons, schools and universities to prevent radicalisation.
In other developments:
:: Authorities in Tunisia have also released images of two men being sought in connection with the shooting, who have been named as Bin Abdallah and Rafkhe Talari.
:: The gunman was linked in reports to Saifallah Ben Hassine, a fanatic who was based in London for at least three years and a disciple of the cleric Abu Qatada.
:: Claims emerged that Rezgui was part of a five-man terror cell which has been in existence for four years. A fellow student told Sky News that he "loved everything (Islamic State) stood for".
:: The first RAF flight carrying the bodies of Britons killed in the Tunisian beach massacre will arrive at Brize Norton on Wednesday.
:: The number of Britons confirmed dead reached 24, with the death toll expected to reach 30, while more victims from the UK were named.
:: Police, soldiers, emergency services and intelligence officials were taking part in London's largest counter-terrorism exercise.
A group understood to be relatives of some of the British dead were among scores of people, local and European, who left flowers and messages at the memorials to the victims on the beach at Sousse today.
The group, escorted by security guards, left bouquets with a message saying: "Taken too soon, missed by many, always in our thoughts, lots of love, Denise, Paul, Mark, Kelly, Lee."
Armed police continued to patrol the beach in front of the five-star hotels, which are are now almost empty at what should be a peak part of the tourist season.