The British government says the Russian plane that crashed in Egypt after taking off from Red Sea resort of Sharm al-Sheikh might have been brought down by an explosive device.
"While the investigation is still ongoing we cannot say categorically why the Russian jet crashed," Prime Minister David Cameron's office said in a statement.
"But as more information has come to light we have become concerned that the plane may well have been brought down by an explosive device."
As a precautionary measure, it said, the government had decided that flights due to leave the resort for Britain on Wednesday evening would be delayed to allow time for a team of UK aviation experts, currently on their way, to make an assessment of the security arrangements in place at the airport.
That assessment was expected to be completed later and there were no more flights due to take off from Britain to Sharm al-Sheikh yesterday.
"We would underline that this is a precautionary step and we are working closely with the airlines on this approach," the statement said.
Extra consular staff were being sent to the resort to help British holidaymakers there, it said, while those either there or planning to travel were advised to contact their airline or tour operator.
Britain was not changing the level of its travel advice to the area, it added.
US says bomb likely cause
An unnamed United States official has told Associated Press they had reached the "tentative conclusion", after intercepting communications, that a group affiliated with Islamic State had planted an explosive device on the plane.
"A bomb is a highly possible scenario," another US official told the AFP news agency.
But they stressed that forensic evidence, including the flight recorders, was still being analysed.
The Egyptian president Abdul Fattah al-Sisi earlier dismissed as "propaganda" claims that extremists linked to IS brought down the aircraft.
And foreign minister Sameh Shoukry called the UK response premature and unwarranted, saying Egypt had taken "exceptional measures" to enhance security at Sharm el-Sheikh airport.
The Russian-operated Airbus A321M crashed on Saturday shortly after taking off from the resort on its way to St Petersburg, killing all 224 people on board.