The Russian airliner which crashed in the Sinai desert, killing 224 people, was downed by a bomb, analysis of the black box recorders indicates.

Sources close to the investigation said an explosion could be heard on the recorder, and ruled out mechanical failure or crew error as explanations.

British and American intelligence sources believe that a bomb was put inside the plane’s hold at Sharm prior to take-off. The Kremlin had previously accused the UK of acting prematurely by suspending flights before the investigation of the crash site and the black box recorders was complete.

But Russia’s reversal appears to place beyond any doubt that the A321 Airbus was destroyed by a bomb, in what would be one of the deadliest terrorist outrages since 9/11.

The distinct sound of an explosion mid-flight could be heard on the black box recorder, a source close to the investigation told France 2. Unnamed officials meanwhile told Le Point that “exterior action” was the only explanation for the crash. An Egyptian government source told ABC that a bomb was now “the most plausible scenario”.

Brits stranded in Egypt

All British flights from Sharm were suspended on Wednesday evening, after Downing Street said new evidence had come to light to indicate the crash may have been a terror attack. Rescue flights to bring home the 20,000 UK citizens stranded in Sharm began, but were immediately beset by delays and disruptions. Only eight of 29 planned flights to the UK flew from Sharm today.

John Casson, the UK’s ambassador to Egypt, who was confronted by dismayed holidaymakers during a visit to the airport, denied reports that Egyptian authorities had been blocking the arrival of empty planes from the UK.

British and Russian passengers queue to leave Sharm el-Sheikh after all flights were grounded (Reuters)

UK Government sources described the airlift as “a hugely complex operation”. A number of empty planes en-route from the UK to Sharm had to diverted after being told there would be nowhere for them to land at the airport. Passengers have been told to carry only cabin baggage, amid concerns around the security procedures for handling luggage, with Egypt blaming pile-ups of suitcases, and a broken down plane, for the disruption.

Russia will now begin the process of airlifting its own citizens out of Egypt. Reports suggest the country has around 50,000 tourists there, many of them in Sharm.

While US and European intelligence sources have cautioned that there was no conclusive “forensic” proof that a bomb downed the plane, any other explanation is now considered unlikely.

Isis affiliates in the Sinai peninsula had previously claimed responsibility for the crash, but had not explained how they downed the plane.

While an investigation by Egypt and Russia of the crash site and the black box recorders is yet to conclude, British and US spies are understood to have identified a bomb as the likely cause, following interception of “chatter” from suspected militants.

The Russian decision followed advice from the director of the FSB, Aleksandr Bortnikov. Dmitry Peskov, spokesman for President Vladimir Putin, said that while the decision to halt Egyptian flights was taken in the interests of safety, it did not mean that terrorism was the cause of the crash of flight A321.

Russian passenger plane crashes in Egypt Show all 20 1 /20 Russian passenger plane crashes in Egypt Russian passenger plane crashes in Egypt Egyptian soldiers collect personal belongings of plane crash victims at the crash site of a passenger plane bound for St. Petersburg in Russia that crashed in Hassana, Egypt's Sinai Peninsula Russian passenger plane crashes in Egypt Egyptian soldiers collect personal belongings of plane crash victims at the crash site of a passenger plane bound for St. Petersburg in Russia that crashed in Hassana, Egypt's Sinai Peninsula Russian passenger plane crashes in Egypt In this Russian Emergency Situations Ministry photo, made available on Monday, Nov. 2, 2015, showing Metrojet Airbus A321-200 flight 7K9268 flight recorder on display at an undisclosed location in Egypt Russian passenger plane crashes in Egypt Mourners lay flowers at Pulkovo International Airport outside St. Petersburg. Russia on 1 November mourned its biggest ever air disaster after a passenger jet full of Russian tourists crashed in Egypt's Sinai, killing all 224 people on board. Flags were at half mast on the parliament building, in the Kremlin, and on other official buildings in honour of the victims, most of whom were from Russia's second-largest city of Saint Petersburg Russian passenger plane crashes in Egypt People pay their respects at the entrance of Pulkovo airport outside St. Petersburg, during a day of national mourning for the plane crash victims Russian passenger plane crashes in Egypt Debris from the plane crash in Egypt Russian passenger plane crashes in Egypt A piece of an engine of Russian MetroJet Airbus A321 at the site of the crash in Sinai, Russian passenger plane crashes in Egypt The crash site debris Flight 7K9268 crashed in the Sinai peninsula, in all probability killing every one of the 224 people on board AFP/Getty Russian passenger plane crashes in Egypt The crash site debris Debris lies strewn across the sand at the crash site EPA Russian passenger plane crashes in Egypt Relatives in St Petersburg Relatives react after a Russian airliner with 217 passengers and seven crew aboard crashed, as people gather at the Kogalymaviaís information desk at Pulkovo airport in St Petersburg on 31 October AP Russian passenger plane crashes in Egypt Relatives in St Petersburg A relative of a passenger of MetroJet Airbus A321 at Pulkovo II international airport in St Petersburg, Russia, 31 October 2015. EPA Russian passenger plane crashes in Egypt The plane's journey The plane's last recorded radar position above the northern Sinai peninsula Flightradar24 Russian passenger plane crashes in Egypt Where it crashed A satellite view from Google Maps of the rough area where the plane crashed, in the mountainous Hassana region of the Sinai peninsula. Google Maps Russian passenger plane crashes in Egypt The plane The Metrojet's Airbus A-321 with registration number EI-ETJ that crashed in Egypt's Sinai peninsula REUTERS/Kim Philipp Piskol Russian passenger plane crashes in Egypt The plane The crashed Airbus A321 at Domodedovo international airport, outside Moscow,, on 20 October Russian passenger plane crashes in Egypt Relatives at in St Petersburg A relative of a passenger on MetroJet Airbus A321 at Pulkovo II international airport in St Petersburg EPA Russian passenger plane crashes in Egypt Relatives at in St Petersburg Relatives of passengers of MetroJet Airbus A321 at the Crown Plaza hotel in St Petersburg EPA Russian passenger plane crashes in Egypt Bodies being repatriated An Egyptian soldier prays as emergency workers prepare to unload bodies of victims from a police helicopter to ambulances at Kabrit military airport on 31 October. AP Russian passenger plane crashes in Egypt Bodies being repatriated Ambulances line up as emergency workers unload bodies at Kabrit military airport, 20 miles north of Suez, on Saturday AP Russian passenger plane crashes in Egypt Bodies being repatriated Egyptian paramedics load the corpses of victims into a military plane at Kabrit military air base by the Suez Canal on October 31, 2015 AFP/Getty Images

“As before, there is no dominant explanation because there is no conclusive proof in support of one version,” Mr Peskov said, according to Interfax. Russian officials said that the ban would remain in place for as long as it was required for safety reasons.

Russian forces are engaged in a deepening military campaign in Syria. Confirmation that Isis was behind the attack on its citizens could be damaging for President Putin and may also prove devastating for Egypt’s tourism industry.

Russia’s state tourism agency, Rosturism, said it had set-up an emergency headquarters to coordinate evacuations, and it was reported that Aeroflot had begun sending empty planes to bring back Russian citizens.