A British passenger onboard the Ukrainian airliner that crashed in Iran, killing all 176 onboard was not supposed to be on the doomed jet, but fought for the last-minute seat after his original flight was cancelled.

Sam Zokaei, 42, a British Petroleum engineer, was visiting his mother who had undergone surgery in Iran. Keen to return to London he switched tickets, a close friend told The Independent.

The fateful decision would end his life.

The news came as teenager Arad Zarei was named as the fourth British victim in the crash which took place minutes after Ukrainian International Airlines Flight 752 took off from Imam Khomeini International Airport on Wednesday morning.

Arad Zarei, 17, who had recently relocated to Canada having attended St Mary’s Primary School in Twickenham until 2014, was said to have been visiting his mother in Iran.

Video purports to show moment Ukrainian airliner is hit by missile over Iran

The pair were among four Britons killed on the plane, which Boris Johnson and Justin Trudeau, the Canadian prime minister, have said was likely to have been shot down by an Iranian anti-aircraft missile by accident.

The two other British victims were named as Saeed Tahmasebi, 35 a chartered engineer from Chiswick, west London, and Mohammed Reza Kadkhoda-Zadeh, 40, who ran a dry-cleaning business in Brighton.

“I was really shocked. I knew Sam was flying back on that day but was not sure which flight. The first thing I did was to text him, but the text never got to him,” said Mahmoud, a British friend who lived with Mr Zokaei for five years and worked briefly with him at BP. Mahmoud did not provide his surname.

“He was not even supposed to be on that flight. His flight got cancelled and he managed to get himself on this flight to come back,” the friend added.

Several flights were reportedly cancelled the night the jet crashed, amid heightened security tensions.

Hours before Flight 752 took off at 06.13 [03.13 GMT], Iran had fired a volley of ballistic missiles at airbases in Iraq housing US troops.

Tehran’s strikes were retaliation for an American drone strike on Baghdad International Airport last Friday which killed Iranian top general Qassem Soleimani.

Mr Johnson said on Thursday night 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”.

His statement came after US officials and Canadian prime minister Justin Trudeau said evidence pointed to an Iranian missile downing the jet accidentally.

The Iranian authorities have repeatedly denied the missile theory and have instead pointed to engine failure as the cause.

All 176 onboard including the crew and 15 children were killed in the crash. Among the dead were 82 Iranians, 63 Canadians, 11 Ukrainians, and 10 Swedes.

Mahmoud told The Independent that Mr Zokaei was in the Iranian capital as his mother had eye surgery there and he was visiting her in hospital. His original flight had been cancelled leading him to find an alternative route home.

An avid skier, hiker and traveller he left behind one sister and his parents.

His family and friends are “really devastated,” Mahmoud continued.

“He cared a lot for his friends. He was someone that was a key figure in the lives of many … He was also a very generous person with his finances. For me Sam was not just a friend but also family,” he added.

Meanwhile, in Twickenham, Arad Zarei’s classmates were given the day off school to mourn their friend, who had kept in touch on social media.

Zarei was apparently planning on coming back to the UK to celebrate his 18th birthday and had hoped to study at a British university, The Times reported.

A spokesman for St Mary’s School said the staff and governors were “shocked and immensely saddened” to hear of the pupil’s death.

“Arad attended St Mary’s until 2014 and is remembered fondly by his teachers. We wish to extend our prayers and condolences to Arad’s family and friends at this time,” the statement read.

Tahmasebi, 35, who worked with Laing O’Rourke in Dartford, was onboard the plane with his new wife Niloufar Ebrahim. They had married just a month before in the UK and were in Iran for he second leg of their wedding.

Tahmasebi, a research PhD candidate at Imperial College, was planning to settle in London with his partner who was an Iranian citizen.

They were only travelling that day because they stayed behind in the Iranian capital after guests had flown home to pick up wedding photos, The Telegraph reported.

The fourth British victim Mr Kadkhoda-Zadeh, who runs Smarty’s dry-cleaning business in Hassocks, was reportedly visiting family in Iran over the Christmas period, and like the other British victims was travelling back to the UK via Ukraine.

Sussex-based paper The Argus said that tributes had been left outside his shop.

He was a divorced father of a nine-year-old girl, who was described as “quiet, polite and lovely”, the paper added.

Friends of high school student Arad Zarei hold his pictures as mourners attend an outdoor vigil for the victims of a Ukrainian passenger jet which crashed in Iran (Reuters/Chris Helgren) (REUTERS)

Among the other victims was Toronto resident Sheyda Shadkhoo who had a “premonition” the plane was going to crash and posted on Instagram a selfie and poem expressing her fears, her husband said.

Speaking to Canadian broadcaster CBC, Hassan Shadkhoo said his wife was concerned something might happen and called him 20 minutes before the plane took off for reassurance.

“She was an angel,” he told CBC. “I wish I didn’t exist right now.”

Ottawa resident Mohsen Ahmadipour, narrowly escaped death when his ticket for Flight 752 was declared invalid at Tehran airport. However, his wife Rosa Azadian, who was travelling with him, boarded the plane anyway as they planned to reunite in Canada.