A total of 38 people, including up to 30 Britons, died after 23-year-old student Seifeddine Rezgui opened fire in the resort of Sousse on Friday.
Downing Street has confirmed that 21 British victims have died, with the number expected to rise to about 30 in the coming days.
The RAF said that their C17 aircraft was "specially modified for this mission" to allow it to carry additional stretchered patients.
All of the injured Britons have now been returned to the UK, and are being treated in the Queen Elizabeth hospital in Birmingham, the John Radcliffe hospital in Oxford, Derriford hospital near Plymouth and St Mary's hospital in London, Downing Street announced.
The C17 aircraft landed at Birmingham Airport last night carrying some of the British victims
Repatriation of the bodies of those killed will start to be repatriated to RAF Brize Norton tomorrow, if families decide they want the Government to bring their loved ones back to the UK.
David Cameron updated the Cabinet today on the situation in Tunisia and its aftermath, with Defence Secretary Michael Fallon, Home Secretary Theresa May and Foreign Secretary Philip Hammond all taking part in the discussion.
There are 27 British security, Foreign Office and military officials were now on the ground in Tunisia.
Hammond is due to chair a fresh Cobra meeting in Whitehall this afternoon to further coordinate the British response to the murders.
It comes as Cameron announced there will be a nationwide minute's silence on Friday.
The Union flag will be flown at half mast from Buckingham Palace as well as all Whitehall departments on Friday, to mark a week since the terror attack, No.10 added.
The RAF aircraft landed in Birmingham Airport on Monday night. There were highly-trained RAF medical staff were on the flight, including consultant anaesthetists, an anaesthetic registrar and intensive care nurses.
Squadron leader Adam Manson, helping to coordinate the aeromedical evacuation effort, said: "The C-17 allows us to bring back multiple critically-injured patients safely and quickly on one aircraft.
"Having medically-trained personnel onboard also helps us organise the patients’ onward moves to the appropriate major trauma centres around the UK.
"The RAF is well-versed in aero-medical missions, having performed the role in Afghanistan. The injuries the patients have sustained are very similar to the types of injuries the military can face and the teams we’ve sent are used to treating patients on aircraft – they’re in the best possible hands."
Inside the RAF's C17 aircraft
One of those being transported home is Gina Van Dort, who heartbreakingly clutched the body of her deceased husband Chris Dyer when they were shot fleeing the gunman on Friday.
The 30-year-old from Watford has been left with serious facial injuries after she was shot under the chin during the attack.
A doctor who tended to Ms Van Dort said the widow communicated with staff by writing a note asking for her wedding ring, which had been removed before surgery, The Daily Mail reports.
Speaking of the moment she found her at the hotel, Dr Hajer Kraiem said: "Her husband was dead. I went to the hotel and there were three victims dead and then I saw Gina, she was hugging her husband.
"She didn’t want to leave him. When we tried to bring her [to the ambulance] she held tighter. Maybe she didn’t know he was dead.
"She had major facial trauma and she couldn’t speak. She was conscious [but] she couldn’t speak, [the sound] was incomprehensible."
Gina Van Dort is pictured with her husband Chris Dyer at this year's Brighton Marathon
Cameron said that Britain must become "intolerant" of extremist Islam in order to protect "the very things we stand for".
The Prime Minister announced on Monday that the victims of the Sousse attack will be returned to the UK within 24 hours.
He also said there will be a nationwide minute's silence at midday on Friday.
Speaking of the minute's silence, he told MPs: "I know the whole country will want to share in a moment of remembrance."
The Prime Minister told the Commons: "This is not the war between Islam and the West which Isil want people to believe - it is a generational struggle between a minority of extremists who want hatred to flourish and the rest of us who want freedom to prosper - together we will prevail."
The father of the Tunisian gunman spoke of his devastation and shame at his son's actions. Hakim Rezgui spoke about his son to ITV News and said: "I am so shocked... I feel like I have died along with the victims - I am so ashamed."
Home Secretary, Theresa May, visited the site of the Tunisian massacre yesterday. She said the attack was "a despicable act of cruelty".
She added: "How could a place of such beauty, of relaxation and happiness be turned into such a scene of brutally and destruction."
Theresa May visited the site of the terror attacks in Tunisia on Monday
May held talks with Tunisian, German, French and Belgian ministers on addressing the threat posed by IS.
Echoing Cameron's sentiments, May said: "We are very clear that the terrorists will not win. We will be united in working together to defeat them, but united also in working to defend our values."
The Tunisian interior minister Mohamed Gharsalli said a "significant number" of individuals allegedly linked to gunman Rezgui have been arrested.
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