Boris Johnson said there was now a “body of information” that indicated the Tehran airliner crash was caused by an Iranian missile.
The British Prime Minister made the announcement after US officers and the Canadian Leader said Iran looked to have been involved in the downing of the Ukrainian International Airlines Flight 752.
In a statement, Mr Johnson said: “There is now a body of information that the flight was shot down by an Iranian surface to air missile. 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 also confirmed four Britons died in the plane crash, up from the earlier reports of three.
The Conservative Party leader said the Government was “providing support to their families at this most terrible time”.
The three known British victims included engineer Sam Zokaei, from Surrey, Saeed Tahmasebi Khademasadi, from west London, and Mohammad Reza Kadkhoda Zadeh, from near Brighton.
In total 176 people were killed when the plane crashed moments after it left Imam Khomeini International Airport in Tehran at 6.10am local time (2.40am GMT) on Wednesday, bound for the Ukrainian capital Kyiv.
At least a third of the deceased held Canadian passports and Prime Minister Mr Trudeau told a press conference that the news of a missile being responsible was likely to comes as a “shock” to the grieving families.
He made his comments after two US officials said it was “highly likely” an Iranian anti-aircraft missile brought down the passenger plane.
Boris Johnson had used a phone call with Ukrainian President Volodymyr Zelenskyy on Thursday to call for a “full, credible and transparent investigation” into what happened.
Ukraine’s Secretary of the National Security and Defence Council, Oleksiy Danilov, posted on Facebook that a missile attack was one of the possible causes of the fire that caused Ukraine International Airlines’ (UIA) PS752 aircraft to break-up as it gained height after taking off from Tehran airport on Wednesday.
The Iranian military had disputed suggestions the airliner was brought down by a missile, with officials in Iran blaming an engine fire.
But Mr Danilov said “information about the detection of the shards of a Russian missile” – understood to be a Russian Tor M1 missile – meant an attack on the plane could not be ruled out.
The Prime Minister pushed for the facts to be established during his conversation with Mr Zelenskyy on Thursday.
Mr Zelenskyy ordered a criminal investigation soon after the details of the tragedy emerged this week.
He has invited United Kingdom aviation experts to join the investigation into the crash.
A spokesman for the president said: “The President invited the United Kingdom to join the investigation.
“Boris Johnson supported this idea and stressed that the best British experts should be involved in clarifying all the circumstances of the tragedy.”
The airline ruled out human error in the aftermath of the incident, and crew were not said to have made an emergency call.
The crash comes amid escalating tension in the Persian Gulf following the killing last week of Iranian general Qassem Soleimani by the US, prompting several airlines to reroute flights away from Tehran’s airspace.
Iran fired missiles at army bases in Iraq were both US and UK troops were stationed, although both Number 10 and the White House said there had been no casualties incurred.
Mr Johnson, in his statement, added: “The UK continues to call on all sides urgently to deescalate to reduce tensions in the region.”