Prime minister Boris Johnson, echoing similar statements from Canadian president Justin Trudeau and US president Donald Trump, said there was now a "body of information" suggesting Ukraine International Airlines (UIA) flight PS 752 "was shot down by an Iranian surface to air missile".

All three leaders have through stressed the incident "may well have been unintentional". The tragedy unfolded just hours after Iran launched retaliatory missile strikes on two Iraqi military bases housing US and coalition personnel following the death of top general Qasem Solemeini, who was killed in a US airstrike on Friday (3 January).

All 176 people, 167 passengers and nine crew, died when the plane burst into flames and crashed on the outskirts of Tehran. Onboard were four British nationals, Johnson confirmed, as well as 63 Canadian nationals, many of whom were expecting to transit through Kiev where the flight was heading and travel onwards to Toronto.

Johnson described the incident as a tragedy and said his thoughts were with all of those who had lost loved ones. He added the government was providing support to their families.

"We are working closely with Canada and our international partners and there now needs to be a full, transparent investigation," said Johnson. "The UK continues to call on all sides urgently to de-escalate to reduce tensions in the region."

Foreign secretary Dominic Raab echoed Johnson’s comments during a meeting with Canadian foreign minister Francois-Philippe Champagne on Thursday (9 January). "We agree with the Canadian assessment that indicates that Ukraine International Airlines flight was shot down by a surface to air missile.

"Our view on the crash underlines why we urgently now need an independent, full and transparent investigation to establish what caused it. The Iranian regime must open up to the international community, including access to the crash site, so we can get the truth as quickly as possible and give the families of the victims an understanding of what happened to their loved ones."

Citing Iranian state media, the Association Press reports Iran has now invited Boeing to take part in the investigation. Immediately after the incident, which both Iran and Ukraine initially said was the result of a technical issue with the Boeing 737-800 aircraft, it was reported Iran would deny the US and Boeing access to the plane’s black box, owing to the ongoing tensions with the US following the killing of Solemeini.

Early on Friday (10 January), the FCO updated its travel advice for Iran to advise against all travel to Iran and all air travel to, from or within the country. "If you’re in Iran, you should review your departure options and consider leaving the country," said the FCO.

Its full statement on the situation read:

"There are heightened tensions in the region. On 8 January 2020, Iran fired missiles against two military bases in Iraq containing US personnel. Tensions between Iran and other countries could escalate rapidly. Anger inside Iran is high, following the death of Iranian General Qasem Soleimani in a US strike in Baghdad on 3 January. There is a possibility of an increased threat against Western interests and the security situation could worsen with little warning.

"There is uncertainty surrounding the crash on 8 January of a Ukraine International Airlines flight shortly after take-off from Imam Khomeini International Airport in Tehran. If you decide to travel by air against FCO advice, contact your airline or travel company for the latest information before travelling. Flight schedules may be subject to cancellation at short notice. There are alternative land and sea-based routes to leave Iran.

"In the event of a sudden deterioration in the security station, there may be limits to the assistance the FCO can provide, depending on the security and transport situation. You should not assume that the FCO will be able to provide assistance to leave the country.

"If you decide to remain in Iran, you should maintain a low profile and keep up to date with developments, including via this travel advice. Avoid any rallies, marches and processions, keep away from military sites and follow the instructions of the local authorities at all times.

"There is a risk that British nationals, and a significantly higher risk that British-Iranian dual nationals, could be arbitrarily detained or arrested in Iran. The criminal justice process followed in such cases falls below international standards. If you are a dual British-Iranian national and are detained in Iran, the FCO’s ability to provide consular support is extremely limited."