David Cameron is chairing a meeting of the government's emergency Cobra committee on Friday morning to discuss Britain's response to the shooting down of the Malaysia Airlines jet over Ukraine.
The prime minister is also planning to discuss the crisis with other international leaders – including possibly Barack Obama, and Vladimir Putin – and he has rescheduled a cabinet meeting that was due to take place in Chequers so that it will be held in London instead.
At the Cobra meeting, due to start at 10.30am, Cameron, senior ministers and intelligence officials will consider the latest evidence about what is known about the cause of the crash, as well as receiving an update about the Britons involved.
A cross-Whitehall meeting on Thursday night concluded that there were a range of possible explanations for the downing of the Malaysian jet and that it was impossible to be "100% sure" at that point exactly what had happened, according to a Downing Street spokeswoman.
She said: "One of the issues for Cobra will be: has our information moved on? This is a very serious incident if indeed the plane was brought down by a missile and we will not rush to conclusions.
"The priority is to make sure that we get independent international investigators to the site in time for them to be able to assess what happened."
Malaysia Airlines has said that nine Britons were on board flight MH17 but, according to Downing Street, this could be a "slightly changing picture" because there were some passengers on the flight whose nationalities were not yet known.
Britain was instrumental in pushing for a meeting of the United Nations security council to discuss the crisis. Cameron has also been at the forefront among EU leaders in pushing for tougher sanctions on Russia because of its continuing reluctance to defuse the situation in Ukraine, but the attack on flight MH17 takes the crisis to a new dimension.
At the Cobra meeting, ministers will also discuss support for the families of the Britons killed when the plane crashed, and what efforts will be made to recover and repatriate their bodies.
One victim has been named as Glenn Thomas, 49, a media officer at the World Health Organisation in Geneva. He was one of up to 100 people on the flight on their way to an international Aids conference in Melbourne, Australia.
Paying tribute to Thomas, WHO spokeswoman Fadela Chaib said: "I can confirm he was on the flight travelling to Australia to attend the Aids conference in Australia.
"For the time being we would like to give his family time to grieve. We have lost a wonderful person and a great professional. Our hearts are broken. We are all in shock."
Thomas, a former BBC journalist, was reportedly from Blackpool and had recently celebrated his birthday.
It has also been reported that two Newcastle United fans travelling to see their team play in New Zealand are feared to be among the victims.
The fans' website NUFC.com reported that John Alder and Liam Sweeney were on board flight MH17.
Alder, believed to be in his 60s, and Sweeney, 28, were flying to New Zealand to watch their team play in a pre-season tour.
A tribute on the website said: "Both were well known to away followers, particularly John, whose usual match day attire led to the affectionate nickname of 'the Undertaker'. There has been no official confirmation of the names of any of the nine British passengers on board the Boeing 777.
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