Britain said on Wednesday that 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 on Wednesday.
"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.
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.