A 14-year-old boy from London was among three Britons killed in Wednesday's earthquake in central Italy.
Marcos Burnett was on holiday with his parents and sister when the quake hit.
Will Henniker-Gotley, 55, and his wife Maria, 51, from Stockwell in London, also died.
Both families were staying in the same property in Sommati, a hamlet outside Amatrice, where emergency teams are continuing to search for survivors. At least 268 people lost their lives.
Marcos Burnett's parents suffered from minor injuries and are being treated in hospital in Italy. His sister also survived.
A statement from the Foreign Office said: "It is with sadness that we can confirm the deaths of Maria, 51, and Will, 55, Henniker-Gotley and Marcos Burnett, 14, in the earthquake in Amatrice, Italy on 24 August.
"The families have paid tribute to the tireless work of the Italian rescue workers and hospital staff and expressed their gratitude for the love and support they have received from the Italian people."
The two families were visiting the area for a summer festival.
Nando Bonnani, who lives in the house next door in Sommati, was the first to try to rescue them.
"I shouted and called out for Maria and Will but I didn't hear any reply," he told the BBC. "There was nothing we could do with our bare hands. They were sleeping in a room on the ground floor and it all just collapsed on them."
Jill Meyer, one of Mr and Mrs Henniker-Gotley's neighbours in London said the couple were "lovely dear people".
"We are just deeply shocked... I don't know what has happened to their children. I'm sure they will have been on holiday with them. So I pray, although it would be awful not to have their mother and father, I just hope they have survived it. They were lovely and we will miss them here."
The 6.2-magnitude quake hit at 03:36 (02:36 BST) on 24 August, in a mountainous region 100km (65 miles) north-east of Rome.
At least 268 people are now known to have died and 400 were injured. Teams have continued to search the rubble of toppled buildings for a second night.
Amatrice is one of the worst affected areas, along with the towns of Arquata, Accumoli and Pescara del Tronto.
Marcos Burnett's parents were originally taken to separate hospitals 60km (40 miles) from each other in Rieti and L'Aquila.
Pasquale Carducci, director of Rieti Hospital said: "The British woman was brought here by rescue workers on Wednesday while her husband was taken to L'Aquila.
"When we discovered he was there, we decided they would be happier together, so we decided to reunite them. Since the man was less badly hurt, it was easier to bring him to her. We hope that they can be a support to each other."
Italy has declared a state of emergency in the regions worst hit and Italian Prime Minister Matteo Renzi has pledged €50m (£42m) in funds for rebuilding.
Buckingham Palace said the Queen had made a personal donation to support the work of the Italian Red Cross in the search and rescue effort.
Foreign Secretary Boris Johnson said the UK had also offered to help rescuers and additional consular support staff had been deployed to the region.
He added: "My deepest sympathies are with the Italian people and everyone affected by the terrible earthquake that struck central Italy.
"The British government has offered any assistance that we can to help with the recovery effort and I have spoken with Italian foreign minister Paolo Gentiloni to express my condolences personally."
Hundreds of aftershocks have hit devastated areas of central Italy, hampering the efforts of the 5,000 rescuers.
Another magnitude-4.7 tremor struck early on Friday.
Firefighters and rescue crews using sniffer dogs have been working in teams around the hardest-hit areas of the country.
But they say they no longer have much hope of finding survivors in the rubble.
"We will work relentlessly until the last person is found, and make sure no one is trapped," said Lorenzo Botti, a rescue team spokesman said earlier.
The towns are usually sparsely populated but numbers staying there have been swelled by tourists visiting for summer, making it difficult to estimate the precise number missing.
Little is currently known about the nationalities of other foreign nationals caught up in the quake, but Romania has said five of its citizens have died and 11 are missing, while Spain's foreign ministry says at least one Spaniard has died.