LONDON — British Prime Minister David Cameron declared today it was “more likely than not” a bomb brought down a Metrojet flight packed with Russian vacationers — a scenario officials from Russia and Egypt have tried to dismiss as premature speculation.

Cameron said he had grounded all flights to and from Egypt’s Sinai Peninsula, stranding thousands of British tourists at the Red Sea resort of Sharm el-Sheikh, because of “intelligence and information” indicating a bomb was the likely culprit in the crash Saturday that killed 224 people.

Islamic State group has claimed it brought down the plane in the Sinai, a report rejected by Russian and Egyptian officials as not credible. Egypt is fighting an Islamic insurgency in the area where the plane crashed.

Cameron said he had “every sympathy” with the Egyptians, who rely so heavily on tourism, but added he had to “put the safety of British people first.”

“We don’t know for certain that it was a terrorist bomb ... (but it’s a) strong possibility,” Cameron said at his London office. “There’s still an investigation taking place in Egypt. We need to see the results of that investigation.”

He said he would call Russian President Vladimir Putin later today to discuss the crash.

A British team is working to tighten security at the Sharm el-Sheikh airport with an eye toward resuming flights. Cameron said “we want to start as soon as possible” to bring tourists home, and empty planes will be flying out from Britain to do that, but it would take some time.

British Transport Secretary Patrick McLoughlin said Egypt will have to put in place tighter long-term security measures before British flights will resume flying. He told the House of Commons that British security teams sent to Sharm el-Sheikh “will be working intensively with the Egyptian authorities to allow normal scheduled operations to recommence.”

McLoughlin said short-term measures, including different luggage-handing arrangements, would allow the estimated 20,000 British citizens in the Sharm el-Sheikh area, many of them tourists, to fly home.

Germany’s Lufthansa Group announced today it would cancel flights of its subsidiary airlines — Edelweiss and Eurowings — to Sharm el-Sheikh. Flights to Cairo would not be affected, the company said.

Egypt’s minister of civil aviation, Hossam Kamal, insisted the country’s airports do comply with international security standards and said “the investigation team does not have yet any evidence or data confirming this hypothesis” of a bomb bringing down the plane.

A spokesman for Putin, Dmitry Peskov, insisted aviation investigators were working on all possible theories as to why the Airbus A321-200 crashed in Egypt’s Sinai Peninsula, killing everyone on board. He said naming just one possibility was mere speculation.

“One cannot rule out a single theory, but at this point there are no reasons to voice just one theory as reliable — only investigators can do that,” Peskov told reporters in Moscow.