UK travel to Iran has been banned amid speculation over the cause of the crash of a Ukrainian International Airlines flight shortly after take off from Tehran airport.

All 176 people on board the Boeing 737 were killed with latest evidence suggesting an Iranian missile inadvertently brought down the aircraft – a claim denied today by Iran’s civil aviation authority.

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The 737 crashed just hours after missile strikes were carried out by Iran on Wednesday on two airbases housing US forces in Iraq in retaliation to the US killing of Iranian general Qasem Soleimani.

Canadian prime minister Justin Trudeau said he had received intelligence from multiple sources indicating that the aircraft was shot down by an Iranian surface-to-air missile, adding that it was possible that this was unintentional.

A total of 63 Canadians were on the aircraft who were expecting to fly on from Kiev to Toronto. There were four British passengers on board.

Foreign and Commonwealth Office advice against all travel to Iran was imposed in the early hours of this morning.

The FCO also advised against all air travel to, from and within Iran.

“If you’re in Iran, you should review your departure options and consider leaving the country,” the new guidance said.

The FCO added: “There is uncertainty surrounding the crash on 8 January of a Ukrainian 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.”

And the FCO warned: “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.

“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.”