He has also suggested the attack, which killed 38 people, may have been planned by IS in Syria.
A C-17 military transport aircraft, which left Tunis airport earlier on Wednesday, landed at a RAF base in Oxfordshire.
Foreign Secretary Philip Hammond said that one victim, who is believed to be British, remained to be identified, but the police will require further time to confirm the identity.
A total of 12 people have now been arrested as part of the investigation into the massacre at a beach resort in Tunisia that killed 38 people, including three Irish, officials said today.
Lazhar Akrem, a senior official in Tunisia’s ruling Nida Tounes Party, told Tunisian broadcaster Mosaique Radio that the suspects were being questioned. The first RAF flight to repatriate the bodies of those killed in the attack returned eight of the victims to their loved ones.
Prime Minister David Cameron has promised to back a full investigation, calling for “a response at home and abroad” to violent Islamic fundamentalism. Britain on Wednesday began repatriating the remains of victims of last week’s terror attack at a Tunisian beach resort. They are being treated at hospitals in Birmingham, Oxford, Plymouth and London.
The other Britons brought back yesterday included Carly Lovett, 24, a fashion blogger from Gainsborough, Lincolnshire, who reportedly survived the massacre on the beach only to die when a grenade was thrown into her hotel.
That particular threat was withdrawn three days later, after Isis strategists issued a document pointing out that many Muslims would inevitably be killed if the airline were attacked.
Tunisian authorities have made several arrests following the attack, with some sources claiming that Rezgui had been trained in Libya.
They have said that the gunman Seifeddine Rezgui had a “network” of accomplices who aided in the attack.
In its wake, Tunisian authorities vowed new heightened security measures, including 1,000 armed officers to reinforce tourism police – who will be armed for the first time – at hotels, beaches and other attractions.
A minute’s silence was held in the Dáil on Tuesday as a mark of respect for the victims, while flags flew on Government Buildings at half-mast.
Coroner Chinyere Inyama, sitting at West London Coroners Court, is expected to open and adjourn the hearings, in which a headline cause of death will be confirmed and the bodies will be released.
Mr Mellor, an engineer from Bodmin in Cornwall, was killed as he shielded his wife on the beach.
Charles Evans, known as Patrick, was believed to be 78.