A 2010 plane crash that killed 96 people, including Poland’s president, came after senior officials urged the crew to land in thick fog, while some drank beer on board, according to a leaked transcript of cockpit recordings.

The transcript, released by Polish radio station RMF FM on Tuesday, undermines theories widely circulating in Poland that the crash was caused by Russian sabotage, but it has been disputed by the Polish authorities.

The revelations come as Poland gears up for presidential elections in May and a closely contested parliamentary poll in the autumn, in which the late president’s twin brother will lead the rightwing opposition.

“The fifth anniversary of the crash is on 10 April, and we’re in the middle of an election campaign, so this is either topical or a small grenade thrown into the campaign,” Krzysztof Bobiński, a commentator close to the ruling Civic Platform party, told the Guardian.

A firefighter walking near some of the wreckage at the crash site.

The crash in Smolensk, Russia, killed president Lech Kaczyński and his wife, the governor of the central bank, senior military officials and other high-ranking officials. They were travelling to a ceremony commemorating the 70th anniversary of the Katyń massacre, in which thousands of Polish officers were killed by Soviet secret police.

According to the transcript, the crew repeatedly asked people to be quiet or leave the cockpit. It claims the pilots suggested diverting or turning back due to fog, but were put under pressure to land so that the president did not miss his engagement.

According to the radio station, the head of the Polish air force, Andrzej Błasik, told the pilots “you’ll make it easily”, 41 seconds before the crash. Around 15 minutes before, a senior foreign ministry official told the captain “we will try until we make it”, it is claimed. The transcript also suggests that people in or near the cockpit were drinking beer.

An official from the Warsaw military prosecutor’s office, which is undertaking an ongoing investigation into the crash, said the transcript contained inaccuracies, without specifying which sections were incorrect, Reuters reported.

He added tests had indicated crew members and others cited on the transcript were not under the influence of alcohol.

But RMF has said the transcript was produced after Polish investigators used new technology to transcribe parts of a recording provided by Russian authorities that had previously been unintelligible.

Smolensk air crash – a year on and the scars are yet to heal Read more

“RMF is a top radio station,” a defence industry source with intimate knowledge of Polish politics told the Guardian. “This was a deliberate leak to it during the election campaign. [It’s] all true.”

The findings of both Russian and Polish investigations into the crash, which have largely blamed pilot error, have been disputed by leading members of late president Kaczyński’s party, Law and Justice (PiS), which is currently in opposition.

Parliamentarians have suggested the aircraft was brought down by two onboard explosions, with an implication that Russian foul play may have had a hand in the disaster, in the context of Kaczyński’s anti-Russian stance. That the plane was flying to an anniversary of the Katyń massacre, a wartime event long denied by Moscow, has made the issue even more charged.

Maciej Lasek, chairman of the group of experts tasked with explaining the causes and circumstances of the Smolensk crash, said in a statement issued to the Guardian that the transcript does not fundamentally contradict the findings of the official Miller commission, which investigates accidents involving military aircraft in Poland and issued Warsaw’s official verdict on the crash in 2011.

“The information published today by the media only provides some additional details to the contents of the Miller Commission’s report,” Lasek said. “It does not change the causes of the crash established by the commission, nor the safety recommendations formulated in the report to prevent such accidents in the future, which were the sole purpose of the Miller commission’s investigation.”

The late president’s twin brother, Jarosław, will lead Law and Justice in parliamentary elections in autumn, in which the party is expected to put up a stout challenge against Civic Platform. He has suggested his brother’s death was an assassination, and supporters accuse Platform leadership of a cover-up.

Facebook Twitter Pinterest Lech Kaczyński, left, and his identical twin, Jarosław Kaczyński, right, confer during a 2002 parliamentary session in Warsaw. Photograph: Jacek Turczyk/EPA

“Many people in PiS are very much wedded to the idea that the crash was some sort of plot, rather than an accident,” said Bobiński. “It’s a double tragedy in that nearly 100 people died, and secondly that 30% of the Polish population believe this conspiracy theory.”

He added he did not expect the latest claims to have a great impact on the presidential election, in which Civic Platform’s Bronisław Komorowski is widely expected to win re-election.

“It could have an effect on the parliamentary election at the edges. It just shows those who believe the plot theory to be slightly unbalanced. But it tells us more about the atmosphere at the top of the air force, which has hopefully now been cleared up, than it will feed into the political campaign”