Boris Johnson, the prime minister, said there was “a body of information that the flight was shot down by an Iranian surface-to-air missile” and “this may well have been unintentional”.
“We are working closely with Canada and our international partners and there now needs to be a full, transparent investigation,” Mr Johnson added. He also confirmed that four Britons had died in the disaster, an increase from earlier reports of three British deaths.
His statement came after US officials and Canadian prime minister Justin Trudeau said evidence pointed to an Iranian missile downing the jet.
Amid strident denials of responsibility from Tehran, US officials quoted by multiple media outlets said American intelligence detected radar being turned on and several infrared blips on satellite imaging.
These, the sources asserted, equated to missiles being launched and an explosion on impact. The same sources said they believed the Iranians shot the plane by accident.
Mr Trudeau later echoed that, saying in a sombre press conference: ”Intelligence indicates that the plane was shot down by an Iranian surface-to-air missile. This may well have been unintentional.”
Dominic Raab, the UK foreign secretary, said following a meeting with his Canadian counterpart that the British government "agree with the Canadian assessment".
He added: “The Iranian regime must open up to the international community, including access to the crash site, so we can get to the truth as quickly as possible to give the families of the victims an understanding of what happened to their loved ones.
“The families of the victims deserve to know the truth, and we say that whether they're Canadian families, British families, Ukrainian families, Swedish, German, Afghan."
Images purported to be from the crash site show what appeared to be the remnants of a Russian-made Tor missile. Tehran took 29 of the units from Moscow in 2007 as part of a major arms deal.
Iran’s head of civil aviation, Ali Abedzadeh, was quoted by ISNA News Agency as saying it was “impossible that a missile hit the Ukrainian plane”. “How can a plane be hit by rocket or missile [and then the pilot] try to turn back to the airport?” he said.
But footage purported to show a missile being fired at the Ukrainian airliner has surfaced.
A video obtained by Iranian activist Nariman Gharib appeared to show a projectile scuttling upwards, before a flash, which lights up what appears to be the underside of a jet, followed by the sound of an explosion approximately 10 seconds later. This would suggest the plane was approximately 3500m away from the camera at the time of impact.
Digital investigators Bellingcat say they have preliminarily matched the video to a district of western Parand, near Tehran – a location which would have a clear view of the trajectory of the doomed flight 752.
Donald Trump said on Thursday that the deadly crash could have been a mistake and he did not believe it was a mechanical issue.
“I have my suspicions. I don’t want to say that,” he told reporters at the White House.
“It’s a tragic thing when I see that. Someone could have made a mistake on the other side... Not our system. It has nothing to do with us.”
He said the plane was flying in a “rough neighbourhood”, and that “something very terrible happened”.
Earlier, Ukraine’s top security official said a missile strike was considered to be one of the “main working theories” for the crash. All 176 people on board the plane, including nine crew, were killed.
Oleksiy Danilov, the secretary of Ukraine’s Security Council, said officials were in Iran with a brief to look for any possible missile debris or evidence of a rocket strike.
“A rocket strike, possibly a Tor missile system, is among the main working theories, since there is information on the internet about elements of a missile being found near the site of the crash,” he said.
Philip Ingram, a former British military intelligence officer, explained the possible delay in the US releasing its conclusions by saying that announcing a “working confirmation” could be a way of “releasing very classified info without giving your capability away”.
“If the missile was of a certain size, then the US has a satellite network that will pick up any launch,” he said.
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies