Britain has halted flights to and from Egypt's Sharm el-Sheikh airport over concerns a Russian airliner that crashed in the Sinai peninsula may have been brought down by a bomb.
"There is a significant possibility that the crash was caused by an explosive device on board the aircraft," Foreign Secretary Philip Hammond said after an emergency cabinet meeting on Wednesday (local time).
An investigation is under way to probe the causes of the crash of the Saint Petersburg-bound Airbus A321, which killed all 224 people on board when it plummeted from the sky shortly after taking off from Sharm el-Sheikh airport.
A spokeswoman for Prime Minister David Cameron said information had "recently come to light" that increased concerns the crash may have been caused by a bomb.
The British government's announcement came at a diplomatically awkward moment just ahead of a visit by Egyptian President Abdel Fattah al-Sisi, who has dismissed claims that a bomb could have caused the crash as "unfounded speculation".
The estimated 20,000 British nationals in Sharm el-Sheikh will be helped to leave once extra security measures are put in place in the airport, Hammond said.
He thanked authorities in Egypt, which relies heavily on tourism for its economy, and said the decision had been taken "very reluctantly".
"We have to put the safety and security of British nationals above all other considerations," the foreign minister added.
"When we are in possession of information, we will not hesitate to act on it in order to protect that security."
The decision to suspend flights with immediate effect came after a team of British security experts made an initial security assessment in Sharm el-Sheikh.
Militants from the Islamic State (IS) group have said they were responsible for downing the plane without providing details.
A US official told AFP that such an attack "would be something that ISIL would want to do," using another term for IS.
It would be the first ever IS bomb attack on a passenger plane if confirmed.
Cameron's office said ministers would work "urgently" with airlines and Egyptian authorities to get flights running out of the tourist hub, allowing anyone due to leave or who wanted cut short their trips to return home.
Budget airline easyJet said its flights to and from Sharm el-Sheikh were cancelled on Thursday in view of the government's advice and that its future schedule would be kept under review.
Air France and Lufthansa announced they would stop flying over the Sinai peninsula after the crash, and Ireland's aviation authority has also instructed airline operators not to fly over the area.