Almost 300 people have died after a Malaysia Airlines plane was apparently shot down over Ukraine.
According to reports, nine British citizens were among the 295 people on board Flight MH17 - a Boeing 777-200 travelling from Amsterdam to Kuala Lumpur - which was in transit over the war-torn region when it disappeared from radar screens.
The Foreign Office was unable to confirm the ITV News reports that sources at Malaysia Airlines had said nine Britons were on the flight manifest.
Graphic images showed a pall of smoke, charred wreckage and bodies at the crash scene in eastern Ukraine.
Prime Minister David Cameron said he was "shocked and saddened" by the tragedy, while Ukrainian President Petro Poroshenko described it as a "terrorist act".
Mr Cameron added that officials from across Whitehall are meeting to establish the facts, while a spokesman for the Foreign Office said: "We are aware of the reports and are urgently working to establish what has happened."
Ukraine has been torn apart by internal strife since the overthrow of the Moscow-backed regime of Viktor Yanukovych, with Russian backed separatists already accused by the authorities in Kiev of shooting down military jets with missiles supplied by Russia.
The Ukrainian government immediately blamed the separatists with President Poroshenko declaring the "armed forces of Ukraine did not take action against any airborne targets".
Anton Gerashenko, an aide to the Ukrainian Interior Minister, said on his Facebook page that the plane was flying at an altitude of 33,000 feet over when it was hit by a missile fired from a Buk launcher, a Soviet era surface-to-air missile system capable of taking down a high altitude aircraft.
A similar launcher was reportedly seen by journalists near the eastern Ukrainian town of Snizhne earlier today.
But a spokesman for the rebels said the plane must have been shot down by Ukrainian government troops.
The incident sparked a fresh international crisis and is bound to put more pressure on Russia to rein in the rebels.
In the aftermath of the plane's destruction Russian president Vladimir Putin and his US counterpart Barack Obama spoke by phone.
Najib Razak, the Malaysian prime minister, said: "I am shocked by reports that an MH plane crashed. We are launching an immediate investigation."
The jet would have been flying at high altitude on an intercontinental flight that took it over the region.
As the drama unfolded, several other passenger jets were flying through Ukrainian airspace on one of the main routes from Europe to Asia for air traffic.
Today's calamity will prompt questions about why the route was still being used given previous incidents.
A UK Civil Aviation Authority (CAA) spokesman said: "The Ukrainian authorities are responsible for managing their airspace and the UK or other countries cannot enforce airspace restrictions in the area.
"However, the CAA has previously issued advice to UK airlines on operating in this area and following this incident, Eurocontrol (the European air traffic control body) has issued advice to airlines to plan routes that avoid the area."
This evening, the Department for Transport in London said flights were now being diverted.
A DfT spokesman said: "Flights already airborne are being routed around the area by air traffic control in the region. Pilots around the world have been advised to plan routes that avoid the area by Eurocontrol, the European organisation for the safety of air navigation."
Airliner tracking sites seemed to indicated that traffic was now steering clear of Ukrainian airspace.
A spokeswoman for Virgin Atlantic said two flights this evening would be "slightly diverted".
The two flights are the Heathrow to Dubai route and the Mumbai to Heathrow route.
On Wednesday night the Ukrainian authorities said one of their fighter jets was shot down by an air-to-air missile from a Russian plane and Ukrainian troops were fired upon by missiles from a village inside Russia.
The episodes add to what Ukraine says is mounting evidence that Moscow is directly supporting separatist insurgents in eastern Ukraine who have substantial quantities of powerful weapons.
Ukraine said a military transport plane was shot down on Monday by a missile fired from Russian territory. Security Service chief Valentyn Nalyvaichenko said he had "unconditional evidence" that Russia was involved in downing that aircraft.
In a tweet Malaysia Airlines said: "Malaysia Airlines has lost contact of MH17 from Amsterdam. The last known position was over Ukrainian airspace. More details to follow."
Boeing, who manufactured the aircraft, said: "We are aware of reports on MH17. We're gathering more information.
"Our thoughts and prayers are with those on board MH17, as well as their families and loved ones. We stand ready to provide assistance."
The incident brings tragedy to Malaysia Airlines for the second time this year.
In March, one of its jets disappeared with 227 passengers and 12 crew on board in one of the greatest aviation mysteries of all time.
It would not be the first time a civilian airliner has been mistakenly shot down.
In 1988, an Iran Air flight from Tehran to Dubai was shot down by the US warship USS Vincennes in the Persian Gulf. All 290 on board, including 66 children and 16 crew, died.
In 1983, Korean Air Lines Flight 007 from New York to Seoul via Anchorage was shot down by a Soviet military jet near Sakhalin Island in the East Sea. All 269 passengers and crew were killed. The Soviets initially denied knowledge of the incident but later admitted responsibility, claiming that the aircraft was on a spy mission.
A Labour MP called for the House of Commons to delay its summer recess, which is due to begin on July 22, if it is found that the missile used to down the aircraft was supplied by the Russian government.
Bassetlaw MP John Mann said: "This is a very troubling development which raises serious issues that Parliament will want to consider. It cannot do this if it is not sitting. By delaying our recess we will be able to discuss this matter and give it the attention it merits.
"Two hundred and ninety-five innocent people have tragically lost their lives and their families will want to know how and why this has happened.
"Parliamentarians will want to take stock of these tragic events, given that they have serious implications for the future of Europe."
