Two football fans, two students and a World Health Organization worker were among 10 UK passengers on Malaysia Airlines flight MH17.
Some 298 people were on the Boeing 777 Amsterdam to Kuala Lumpur flight, which crashed in eastern Ukraine prompting claims it was shot down.
A Downing Street spokesman said it was "increasingly likely" separatists near the Torez area had shot down the plane.
British air accident investigators are being deployed to the scene.
The Air Accidents Investigation Branch said it was sending a team to assist with the investigation, while British police officers are also travelling to Ukraine to help repatriate bodies.
Speaking after he chaired a meeting of the Cobra emergency committee on Friday, the UK prime minister described the tragedy as "absolutely appalling, shocking, horrific incident". He said it "cannot be allowed to stand".
British passengers believed to have died in the tragedy include:
- John Alder and Liam Sweeney, who were understood to have been travelling to New Zealand to watch Newcastle United's pre-season tour of the country
- Glenn Thomas, 49, a press officer at the World Health Organization (WHO) and a former BBC journalist
- Ben Pocock, a student at Loughborough University, who had been heading to Australia
- Richard Mayne, a student at Leeds University, originally from Leicestershire
- Cameron Dalziel, who was born in Zimbabwe and lived in South Africa, but was travelling on a British passport
Mr Sweeney's father Barry said he had been unable to find out what had happened via official channels and discovered his son had died from a post on Newcastle United's website.
"I was hurt because I think we should've known," he told the BBC.
"I tried to go through the legal way, ringing all the emergency lines or whatever."
Barry Sweeney said he would rather it had been him on the plane, saying of his son: "I just want everybody to know what a tremendous bloke he is."
A statement from the club paid tribute to Mr Alder and Mr Sweeney, who it said were "two of the club's most loyal supporters".
Club manager Alan Pardew said: "Myself and all the players are deeply shocked and saddened by this terrible news".
Newcastle's players are to wear black armbands for both their games against Sydney FC and Wellington Phoenix in New Zealand.
It has emerged that Mr Thomas had been travelling to Australia to attend an international conference on Aids.
WHO spokeswoman Fadela Chaib said: "For the time being we would like to give his family time to grieve.
"We have lost a wonderful person and a great professional. Our hearts are broken. We are all in shock."
Richard Porter, controller of English services for BBC World Service Group, also paid tribute to ex-colleague Mr Thomas, who he said was a "much loved and respected" journalist.
He said Mr Thomas' interest in journalism was "only matched by his interest in humanitarian issues" and said former workmates were "reeling" from the news.
'Warm and caring'
John Wood, headmaster at Mr Mayne's former school, Dixie Grammar in Leicestershire, described him as "a great all-rounder, good academically and also a keen sportsman".
He added: "The thoughts of all of us at The Dixie Grammar School are with Richard's family and friends at this most difficult time; he will be sorely missed."
Mr Pocock 's family said in a statement that he was travelling to Western Australia as part of university exchange. They added: "He was a gifted academic, talented athlete but more importantly a warm, caring, fun-loving son and brother who had an extremely bright future ahead of him."
On Friday evening, a Downing Street spokesman said "the growing weight of evidence" suggested the plane was brought down by a separatist missile from near Torez.
Prime Minister David Cameron said those responsible must be "brought to account".
The Foreign Office is offering consular assistance to families and has sent extra staff to Ukraine.
At an emergency meeting of the UN Security Council called by Britain on Friday, the UK's ambassador Sir Mark Lyall Grant said "searching questions" had to be asked about Russia's links with armed separatists and called for Moscow to issue an "unequivocal condemnation" of their actions.
The Security Council also approved a statement calling for a "full, thorough and independent international investigation" into the tragedy.
Mr Cameron is also expected to speak to Russian president Vladimir Putin on Friday.
Some nationalities unverified
The Duke of Cambridge also spoke of his "deep sadness" over the crash during a speech at a planned event at Australia House in London.
"For all of us who have lost fellow countrymen and women in the tragedy, words cannot do justice to our sense of loss," he said.
The UK Foreign Office has set up a helpline on 020 7008 1500, while anyone concerned could also text MH17 to +44 7860010026, or fill in an online form.
Flight MH17 came down between Krasni Luch, in Luhansk region, and Shakhtarsk in the neighbouring region of Donetsk - 50km from the Russia-Ukraine border.
Malaysian Prime Minister Najib Razak said "no stone will be left unturned" in the investigation into what happened.
Mr Najib said the passengers and crew had come from many different countries.
He added: "Today, regardless of nationality, we are all united in grief."
The UK Foreign Office confirmed 10 UK nationals were on board.
Latest figures released by Malaysia Airlines show the plane was also carrying at least 189 Dutch nationals, 27 Australians, 44 Malaysians - including 15 crew - and 12 Indonesians.
There were also passengers from Germany, Belgium, the Philippines and Canada, the airline has said.
Ukraine's president has called the loss of the plane an "act of terrorism" as the separatist rebels denied shooting it down.
Meanwhile, leading airlines have announced they are now avoiding airspace surrounding eastern Ukraine.
The Department for Transport said flights that were already airborne were being routed around the area.
British Airways said the safety and security of its customers was its top priority.
It added: "Our flights are not using Ukrainian airspace, with the exception of our once a day service between Heathrow and Kiev.
"We are keeping those services under review, but Kiev is several hundred kilometres from the incident site."
Virgin Atlantic said it was re-routing "a small number of flights".