Tributes have been paid to three British nationals who died when a Ukrainian plane crashed in Iran.
Mohammed Reza Kadkhoda Zadeh, who owned a dry cleaners, BP engineer Sam Zokaei and PhD student and engineer Saeed Tahmasebi were all on board the flight.
They were among the 176 people from seven countries who died in the crash.
Ukraine International Airlines flight PS752 crashed just after taking off from Imam Khomeini airport at 06:12 local time (02:42 GMT).
The airline said the plane underwent scheduled maintenance on Monday.
A Downing Street spokesman said the UK was "working closely with the Ukrainian authorities and the Iranian authorities" over the crash, and there was "no indication" the plane was brought down by a missile.
Iran said it will not hand over black box flight recorders recovered from the plane. Under global aviation rules, Iran has the right to lead the investigation, but manufacturers are typically involved and experts say few countries are capable of analysing black boxes.
As well as the three Britons, the victims in the crash included 82 Iranians, 63 Canadians, 11 Ukrainians - including all of the crew, 10 Swedes, four Afghans and three Germans, Ukraine foreign affairs minister Vadym Prystaiko said.
Rescue teams were sent to the crash site but the head of Iran's Red Crescent told state media that it was "impossible" for anyone to have survived the crash.
Tributes were paid locally to Mr Kadkhoda Zadeh, 40, who ran a neighbourhood dry cleaners in Hassocks, West Sussex, and had a nine-year-old daughter.
Steve Edgington from the pet shop next door said he had known Mr Kadkhoda Zadeh for 14 years, and described him as a lovely, hardworking man who was good at his job and loved by staff.
Savvas Savvidis, 36, who rented a room in Mr Kadkhoda Zadeh's home in Brighton, said he was a "super-nice person".
"It's so sad. Before he left we had a conversation, he told me that he spent all his life working, working really hard, and now finally he wants to start to enjoy life a bit more."
Mr Savvidis described Mr Kadkhoda Zadeh as a humble man who loved his daughter very much.
The dry cleaners closed on Wednesday, with neighbouring businesses telling the BBC that staff were too upset to stay open.
Meanwhile, in a statement, BP said "with the deepest regret" that its employee Mr Zokaei, 42, from Twickenham, was among the passengers.
Mr Zokaei had been on holiday. He had worked for BP for 14 years and was based at the company's site in Sunbury-on-Thames in Surrey.
"We are shocked and deeply saddened by this tragic loss of our friend and colleague and all of our thoughts are with his family and friends," BP said.
A friend of Mr Zokaei, who did not wish to be named, told the BBC they were "still in shock".
"He was a highly accomplished person. Very clever and very friendly. Always smiling and full of positive energy. He will be sorely missed.
"He was always trying new adventures. He cycled and toured Europe on bikes a few times. He also loved travelling to interesting far out places."
Also killed was Mr Tahmasebi, 35, who worked as an engineer for Laing O'Rourke in Dartford.
Last year, Mr Tahmasebi married his Iranian partner, Niloufar Ebrahim, who was also listed as a passenger on the plane.
"Everyone here is shocked and saddened by this very tragic news," said Laing O'Rourke.
"Saeed was a popular and well respected engineer and will be missed by many of his colleagues. Our thoughts are with his family and friends at this most difficult time and we will do all we can to support them through it."
'Humble and generous'
Mr Tahmasebi - whose full name was Saeed Tahmasebi Khademasadi - was also a part-time PhD student at Imperial College London's Centre for Systems Engineering and Innovation.
A spokeswoman for the university said: "We are deeply saddened at this tragic news. Saeed Tahmasebi Khademasadi was a brilliant engineer with a bright future.
"His contributions to systems engineering earned respect from everyone who dealt with him and will benefit society for years to come.
"He was a warm, humble and generous colleague and close friend to many in our community. Our thoughts and sincere condolences are with Saeed's family, friends and colleagues, as well as all those affected by this tragedy."
At Prime Minister's Questions earlier, Boris Johnson and Labour leader Jeremy Corbyn both said their thoughts were with the families of those killed.
A UK Foreign Office spokesman has said: "We are deeply saddened by the loss of life in the plane crash in Iran overnight."
They said it was "urgently seeking confirmation" about how many British nationals were on board and would be supporting any families affected.
Melinda Simmons, British ambassador to Ukraine, said her thoughts are with those affected.
Ukraine's state aviation service has forbidden its national airlines from using Iranian airspace from Thursday, with the restrictions in place until an investigation into the cause of the crash has concluded.
Ukraine's embassy in Tehran and Iranian state television both initially said technical issues caused the crash.
But the embassy later removed this statement and said any comment regarding the cause of the accident prior to a commission's inquiry was not official.
Ukraine said its entire civilian aviation fleet would be checked for airworthiness and criminal proceedings would be opened into the disaster.
The country's president warned against "speculation or unchecked theories regarding the catastrophe" until official reports were ready.
Ukrainian International Airlines said the flight disappeared from radar just a "few minutes" after take-off.
The Ukrainian national carrier said according to preliminary data there were 167 passengers and nine crew members on board but its staff were "clarifying the exact number".
"The airline expresses its deepest condolences to the families of the victims of the air crash and will do everything possible to support the relatives of the victims," a statement said.
The airline, which is investigating the crash, said the aircraft - a Boeing 737-800 - was built in 2016 and had its last scheduled maintenance on Monday.
There was no sign of any problems with the plane before take-off and the airline's president said it had an "excellent, reliable crew".
A statement from Boeing said its "heartfelt thoughts" were with all those affected following the "tragic event".
There are several thousand Boeing 737-800s in operation around the world which have completed tens of millions of flights. They have been involved in 10 incidents, including this crash, where at least one passenger was killed, aviation safety analyst Todd Curtis told the BBC.
This is the first time a Ukraine International Airlines plane has been involved in a fatal crash.