US and UK officials said they have information suggesting a bomb planted by Islamic State militants may have caused the crash of Metrojet Flight 9268, killing 224 people in Egypt on Saturday.
The UK has already suspended all flights to and from Sharm El Sheikh, where the ill-fated plane's last journey originated.
"ISIL-Sinai have claimed responsibility for bringing down the Russian aircraft, they did that straight away after the crash,” said Foreign Secretary Philip Hammond.
"We've looked at the whole information picture, including that claim, but of course lots of other bits of information as well, and concluded that there is a significant possibility," he said on Sky television.
Investigators have in the meantime started analysing data from the plane’s black boxes. While information from the flight data recorder has been successfully downloaded, the cockpit voice recorder was damaged in the crash and extracting the recordings will require extra work.
Russian airline Kogalymavia, which owns the Metrojet brand, has concluded inspections of three of its four remaining Airbus A321s, the same type of aircraft as the destroyed jet. The firm has grounded the A321 fleet following the disaster, pending technical examination.
No problems have been found on the three planes.
"The third plane passed checks at 12:05 Moscow time today," said Oksana Golovina of THC Holding, the company which owns Kogalymavia. "The fourth is now undergoing technical checks in Istanbul and will be checked shortly after it returns to Russia."
Russia’s state transport agency cleared the three checked A321s to return to flight.
Kogalymavia has previously ruled out human error and a technical fault as a possible cause of the crash.
Security experts and investigators also said the plane is unlikely to have been struck from the outside, as in a missile attack. However, heat detected around the plane by American satellites at the time of the crash suggests an explosion.
Sinai Province, affiliated to the Islamic State, claimed responsibility for the attack immediately after the crash. An attack on a Russian jet would be possible as retaliation for Russia’s involvement in Syria on the side of President Bashar al-Assad.
Britain said it was working with airlines and Egyptian authorities to put in place additional security and screening measures to allow Britons in Sharm el-Sheikh to get home, but that would take time and there would be no flights returning from the resort on Thursday.
Late on Wednesday, the Irish Aviation Authority (IAA) directed all Irish airlines not to fly to or from the Sinai Peninsula until further notice. The Russian-operated plane was registered in Ireland and the IAA is taking part in the official investigation into the crash.
Russian and Egyptian authorities have said that any claims a bomb brought down the plane were so far only speculation.