Russian authorities insisted that pilot error was behind the plane crash that killed Polish President Lech Kaczynski, as Polish investigation teams arrived at the crash scene to carry out their own inquiry.

Igor Levitin, the Russian Minister of Transport, said the "black box" flight recorders had been recovered in good condition.

"A group in Moscow has begun the decoding of the recorders, the quality of the recording is good, and there is every chance that we will be able to gain all information about the flight," Mr Levitin said during a meeting with Prime Minister Vladimir Putin.

All the deciphering work is being done in collaboration with Polish officials as Russia is anxious to demonstrate that its investigation is open to prevent conspiracy theories in Poland about the causes of the crash.

Mr Kaczynski was a fierce critic of Russia and relations between the countries had only just begun to improve after years of friction.

"They have shown an unprecedented readiness to do everything possible to help Poland," said Fyodor Lukyanov, editor of the journal Russia In World Affairs.

But he added:"Even if it is confirmed that pilot error caused the crash, there will inevitably be those who say it was the KGB that killed Kaczynski."

Alexander Alyoshin, deputy chief of the Russian Air Force's general staff, said the pilot had ignored several orders from air-traffic control not to land at Smolensk because of thick fog.

The airport is a small, military facility that does not usually accept civilian craft. Aviation experts speculated that the pilots may have been ordered to land by the Polish president.

"It's a clear case of VIP-passenger syndrome," claimed flight safety expert Viktor Timoshkin.

"Air traffic control told him to take the plane to Moscow or Minsk. I'm certain that the pilot will have told the president about this, and got a firm reply that the plane must land in Smolensk."

Russian authorities have agreed to a request from Polish colleagues not to begin the clean-up at the wreckage site until the middle of the week.

Officials in Moscow said that two Polish delegations had arrived in the capital to carry out their own investigations.

One set of 27 experts was headed for Smolensk to study the wreckage of the plane, while another 35 would remain in Moscow, where the bodies of all the victims except the President have been sent.

Yesterday afternoon, the coffin carrying Mr Kaczynski was placed aboard a Polish military jet in a solemn ceremony at Smolensk Airport. Mr Putin saw off the coffin.

Relatives of the dead began arriving in the Russian capital last night, where they would begin the task of identifying the bodies of those who died. Authorities were able to identify 24 victims, but for some DNA tests will be required.

The ageing Tupolev 154 plane plunged into forested land just short of the runway at Smolensk Airport on Saturday morning, killing all 96 on board, including Mr Kaczynski, his wife, and dozens of top political and military officials.

They had been travelling to Smolensk to attend a memorial service at nearby Katyn to commemorate the 70th anniversary of the massacre of over 20,000 Polish officers and others by Soviet forces.

Last Wednesday, Prime Minister Putin invited his Polish counterpart, Donald Tusk, to a memorial service at Katyn, but not Mr Kaczynski, who has been a strong critic of Russia in recent years.

Saturday's event had been organised to allow Mr Kaczynski to mark the anniversary also, and it is possible that Mr Kaczynski ordered the plane to land in Smolensk as any diversion would have meant that he would miss the ceremony.

The Polish press reported that just 36 minutes before the crash, Mr Kaczynski called his twin brother Jaroslaw from aboard the plane, to tell him that everything was going to plan and he would soon arrive in Smolensk.

The few locals who had witnessed the crash said the Tu-154 plane came in at a strange angle through the fog, with one wing tipped towards the ground. The wing clipped trees as it came in far too low, and as a result ended up crashing into the ground around 200 metres short of the runway.

One of the policemen guarding the crash site shortly after the accident said it was a scene of unspeakable horror. "I didn't see anything so horrific, even in Chechnya," he said.

"Everywhere there were parts of the aircraft, fragments of bodies. I only saw a couple of whole bodies. The rest was just bits of human bones caught up in the trees."

There have been more than 60 crashes involving Tu-154s in the last 40 years, six of them in the past five years.

But the head of an aviation plant in Russia that overhauled the Polish presidential plane last year said it had been refitted with electronic and navigation equipment and repairs were carried out to its engines. Alexei Gusev yesterday insisted on Russian state TV that the aircraft had been safe to fly.

- Shaun Walker