Image copyright Reuters Image caption Black box flight recorders from the crash scene have arrived in the UK

Whitehall sources say information has emerged that MH17 crash evidence was deliberately tampered with, as the plane's black boxes arrive in the UK.

The sources said this included moving bodies and scattering the parts of other aircraft among the wreckage of the downed Malaysia Airlines plane.

The Air Accidents Investigation Branch in Farnborough has received the flight voice and data recorders for analysis.

All 298 people on board were killed in the crash in Ukraine on 17 July.

They included 10 Britons on the flight from Amsterdam to Kuala Lumpur.

Missile launch

Whitehall sources also told the BBC that pro-Russian rebels are believed to have discussed plans to hand MH17's two black box flight recorders to Russia.

It is understood that some of these conversations were intercepted by Ukrainian intelligence; the information is considered to be "very persuasive".

Intelligence has also been obtained which strongly indicates that the plane was shot down by an SA-11 BUK missile launched from a separatist-held area in eastern Ukraine.

British accident investigators are attempting to retrieve data from the recorders following a request by authorities in the Netherlands.

The Dutch Safety Board, which is leading the investigation, said "valid data" had been downloaded from MH17's cockpit voice recorder (CVR) which will be "further analysed".

The board said: "The CVR was damaged but the memory module was intact. Furthermore no evidence or indications of manipulation of the CVR was found."

Investigators will examine the other black box, the flight data recorder (FDR), on Thursday.

Half-mast flags

Western leaders have accused Russia of arming separatist rebels in Ukraine, and believe the rebels shot down the Boeing 777-200 airliner with a ground-to-air missile.

Russia has suggested Ukrainian government forces are to blame.

It comes as a group of MPs criticised the fact that UK licences to export arms to Russia had remained in place, despite the conflict in Ukraine.

Downing Street said the UK had not sold arms to Russia since March.

The UK and Dutch flags have been flown at half mast in Downing Street in memory of those who lost their lives.

What are black boxes?

They are two devices - neither of them black - which are designed to survive a crash.

The flight data recorder records operating information from the plane's systems, storing a range of information such as altitude, airspeed, engine power and the pilot's use of the controls.

The cockpit voice recorder, as its name suggests, records sound in the cockpit and can be vital in determining what happened to a crashed aircraft.

The black boxes have been transported to the UK after pro-Russian rebels handed them to Malaysian officials this week.

One of the boxes records technical information on the performance of the aircraft and the other takes down sounds such as pilots' voices and, potentially, an explosion.

A spokesman for the Department for Transport said data from the two devices would be downloaded and sent back to Dutch investigators.

Depending on damage to the black boxes the process should take about two days, the spokesman said.

Jonathan Sumberg, BBC transport reporter, said the British Air Accidents Investigation Branch (AAIB) was one of two "replay units" in Europe with the necessary equipment to listen to the cockpit voice recorder.

Image copyright Reuters Image caption The first planes carrying victims' bodies left Kharkiv airport in Ukraine

Image copyright AP Image caption A total of 200 bodies have so far been accounted for out of 298 victims

BBC reporter Theo Leggett said the black boxes would be taken to a sealed room at the AAIB headquarters in Farnborough where the data would be downloaded.

He said the flight recorder could potentially confirm whether the plane had been hit by a missile.

However, he said information from the cockpit voice recorder could be "relatively limited" as the impact of the explosion "would have occurred pretty much without warning".

Former aircraft accident investigator Tony Cable also told the BBC it was unclear how much information would be recovered from the flight recorders.

"The cockpit voice recorder could conceivably record the sound of shrapnel hitting the aircraft from the missile, which is assumed to be the cause, possibly followed by the sound of the warhead then exploding," he said.

'Commercial use'

Two transport planes, one Dutch and one Australian, have landed in Eindhoven with the bodies of 40 of the victims.

Dutch Prime Minister Mark Rutte and King Willem-Alexander and Queen Maxima were present, on a national day of mourning.

The process of indentifying the victims will take place in the Netherlands.

Meanwhile, MPs have been seeking urgent clarification on UK licences to export arms to Russia.

The cross-party Commons Committees on Arms Export Controls said permits covering the sale of sniper rifles, components for air-to-air missiles, anti-aircraft guns, rocket launchers and combat helicopters remained in force.

A total of 251 licences worth at least £132m remained in force, while 31 licences had been halted or suspended, it said.

The government said it was keeping all licences under review and the majority of licences that remained in place were for "commercial use".