The Russian plane that crashed in Egypt at the weekend "may well have been brought down by an explosive device", Downing Street has said.
All flights between the UK and Sharm el-Sheikh have been suspended on Wednesday evening as UK experts assess security at the Egyptian airport.
Number 10 said flights had been delayed as a "precautionary measure" after "more information has come to light".
Russian Airbus 321 crashed on Saturday, killing all 224 people on board.
"We would underline that this is a precautionary step and we are working closely with the airlines on this approach," a Number 10 spokesman said.
Aviation experts have travelled to Egypt assess security arrangements at the Egyptian airport.
Their findings were considered in a one-hour meeting of the government's Cobra emergency committee, chaired by Prime Minister David Cameron.
A further government statement is expected later tonight.
The Irish Aviation Authority said it has directed Irish airlines not to fly to or from the area until further notice.
Transport Secretary Patrick McLoughlin said the delays would allow UK experts time to make sure "the right security measures are in place" at Sharm el-Sheikh airport.
"We cannot categorically say why the Russia jet crashed but we have become concerned that the plane may well have been brought down as a result of an explosive device," he said.
Egypt's President Sisi is currently in the UK and is due to meet Mr Cameron on Thursday. They spoke on Tuesday, before Downing Street released its statement.
Analysis
Image copyrightAFPBy Paul Adams, BBC world affairs correspondent
With two telling words, "may well", David Cameron's spokesperson has left little doubt that British officials believe the Russian jet was brought down by a bomb.
This contradicts President Sisi's initial insistence that the crash was caused by a technical fault and strikes at the heart of Egypt's precious tourism industry.
It's one thing to suspend flights to a foreign country, but to suspend them from that country and insist that your own technical experts assess security arrangements before flights can resume displays a striking lack of confidence in that country's own security measures.
The continued success of Sharm el-Sheikh as a popular holiday destination depends entirely on the ability of the Egyptian authorities to seal off the southern tip of the Sinai Peninsula from its troubled interior.
The British statement clearly questions that ability.
The BBC's Christian Fraser says there are around 2,000 British holidaymakers currently in Sharm el-Sheikh.
Thomas Cook said it currently has 1,700 holidaymakers in the Egyptian resort, adding that it would "continue to monitor the situation".
EasyJet said it had delayed two UK-bound flights from the resort - one to Gatwick and one to Luton airport - this evening.
"We are doing all possible to keep passengers informed," a spokesman added.
Thomson suspended its flights to and from the resort, including flight TOM 895 out of Manchester Airport which was due to land at 23:30.
A spokesman for British Airways, which is due to fly to Sharm el-Sheikh on Thursday, said: "Things are moving fast and we are waiting for the government to update later in the evening."
The government said it recognised that the latest advice "may cause concern" for Britons already in the holiday resort and for those planning to travel, saying people should contact their airline or tour operators.
Extra consular staff have been deployed to the airport, it added.
Foreign Office travel advice for passengers travelling to Egypt has not changed.
Simon Calder, travel editor at the Independent, said "hundreds" of holidaymakers are due to fly back on Thursday or Friday, saying airlines might fly empty planes to Egypt in order to bring people home.
British holidaymaker Craig Peacock, who has been in Egypt for nine days, said finding out he may not be able to return home is "not the greatest news".
But, he said, postponing flights was "the right thing", adding: "We don't want a repeat of what happened last week".
Another tourist, Sarah Cotterill, from Portsmouth, is at the airport waiting for a flight home, alongside several hundred other travellers.
She said there has been little information and airport staff have "no idea" how long they will be waiting.
Image copyrightSarah CotterillImage copyrightAFPImage caption Sharm el-Sheikh is a popular holiday destination for Britons Image copyrightEPAThe suspension of flights was "slightly humiliating" for Egyptian authorities, BBC security correspondent Frank Gardener said.
Our correspondent said the action comes as there have been "growing suspicions" the Russian plane was brought down by an explosive device, possibly by a bomb smuggled into the plane's cargo hold.
The Metrojet flight bound for St Petersburg from Sharm el-Sheikh came down in Egypt's Sinai desert.
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