Military experts say a small bomb – either with a timer, or triggered by altitude – now appears to be the most likely cause of the Russian plane crash in Egypt’s Sinai desert.

Charles Heyman, editor of annual publication Armed Forces of the United Kingdom, said most analysts had begun to think there was an explosion on board the aircraft.

“Maybe not a huge explosion, but an explosion big enough to actually cause the aircraft to shatter, explode in mid-air and then deposit bits of the aircraft over a large area of ground,” Mr Heyman said.

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He said even a very small bomb smuggled inside someone’s hand luggage would have enormous force at such altitude because the cabin was pressurised.

Russian airline Kogalymavia has said the Metrojet Airbus A321 came down from about 30,000 feet because of “external” factors, with human error or a technical fault now ruled out as the cause.

The carrier’s management confirmed no emergency call was made by the pilots during the flight, saying the crew “totally lost control” and did not attempt to make any contact.

At a press conference, Kogalymavia deputy general director Alexander Smirnov said “some kind of external action” was “the only explanation”.

Missile attack ‘unlikely, but not impossible’

Mr Heyman said it was unlikely – although not impossible – that Islamic State (IS) or other militants in the Sinai could have brought the plane down with a missile.

But IS – if it has missiles at all – is likely to have only shoulder-launched or man-portable missiles, which are not capable of striking a plane above 15,000 feet.

Russia has ordered its own investigation into the crash.

“If they’re going to hit an aircraft at 30,000 feet, they would need a BUK or a SAM 6,” he said.

“Something that is on tracks, and a very large missile with a sophisticated guidance system.

“It’s a large piece of equipment that you wouldn’t be able to hide very easily.”

It was a BUK missile – effectively an upgraded version of the SAM 6 – that investigators concluded brought down the Malaysian Airlines plane MH17 from a cruising altitude over Ukraine in July 2014.

Mr Heyman said any such missile in the Sinai could only realistically be sourced from the Egyptian army, which he said was highly unlikely.

“The percentage chance of that happening is quite small,” he said.

“The Egyptians would know immediately that they’d lost it.

“ISIS would have to drive it around, and it would have a signature that you could pick up quite easily. And you would have to have people who are trained to use it.”

Burn marks point to bomb explosion

Mr Heyman said evidence of burning on the rear of the plane bolstered the theory that a bomb brought the airliner down.

“There are apparently some scorch marks, evidence of burning, towards the tail end of the aircraft,” he said.

Andrew Davies, a senior analyst for defence capability at the Australian Strategic Policy Institute in Canberra, agreed that it was unlikely to be a missile that brought the Russian airliner down.

“If the reports are right that the aircraft came down from 30,000 feet, that’s well above the range of shoulder-launched anti-aircraft weapons,” Mr Davies said.

But he acknowledged it was not impossible a shoulder-length missile had struck the plane soon after takeoff.

Mr Heyman said the size of the debris field also supported the bomb theory.

“If the aircraft had just come down of its own volition and crashed into the ground, the area of wreckage would be much less than that — well under half a square mile or so. It does look like there’s been an explosion,” he said.

Mr Heyman said concerns about security at Sharm el-Sheikh airport also suggested it might be more vulnerable than other airports to such an attack.