Prime Minister Boris Johnson has joined other world leaders in suggesting an Iranian surface-to-air missile brought down a plane near Tehran, killing 176 people.
Mr Johnson says there is now "a body of information" indicating the Ukrainian International Airlines Flight 752 was "shot down" by Iran - adding that such an act "may well have been unintentional".
Four Britons were on board the aircraft, which crashed shortly after Iran launched air strikes against Iraqi air bases containing UK and US personnel.
Downing Street is looking into "very concerning" reports from the event, as US officials said it was "highly likely" an Iranian anti-aircraft missile brought down the passenger plane.
Canada's Prime Minister Justin Trudeau said in a press conference that evidence indicated an Iranian missile brought down the plane and said it may have been unintentional.
At least a third of the 176 people on board who were killed had Canadian passports, while four were British.
The UK Foreign Office is now advising against "all air travel to, from and within" Iran, while state TV reports suggest Tehran has dismissed claims a missile hit the plane.
The UK Prime Minister is now "working closely with Canada and our international partners", using a phone conversation with Ukrainian President Volodymyr Zelenskyy to call for a "full, credible and transparent investigation" into the event.
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.
In his social media post, he said any investigation would have to also probe whether the plane had been subject to a terrorist attack by someone on-board the flight, or whether it collided with another object or an engine fire occurred for "technical reasons".
The Ukraine International Airlines (UIA) 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.
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 the 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.
Several airlines have rerouted flights away from Tehran's airspace, amid growing tensions in the region.
Cover image: Wreckage at the site of a crash of an Iranian Boeing 737 jet about 30 miles south of Tehran, Wednesday, January 8, 2020 (Picture: PA).