• The then-business secretary allowed filmmaker Hannah Rothschild to shadow him for eight months leading up to May • Guardian has exclusive footage

A fly-on-the-wall documentary about Peter Mandelson, filmed in the dying days of the Labour government, is expected to cause a political sensation when it is screened later this month.

The Guardian has been given an exclusive preview of the film, which includes remarkably candid footage of sensitive meetings with Labour's high command as the election campaign unravelled, as well as private telephone calls with Gordon Brown and the editor of the Times, James Harding.

Mandelson allowed the filmmaker Hannah Rothschild – sister of financier Nathaniel Rothschild – to film his inner circle in the eight months leading up to the general election in May.

Mandelson ran into controversy in 2008 when he stayed in Corfu on the yacht of Russian aluminium tycoon Oleg Deripaska as a guest of Nathaniel Rothschild.

On the same holiday Mandelson was accused of dripping "pure poison" into George Osborne's ear about Brown.

Hannah Rothschild's access was unfettered: Mandelson is shown at home in his dressing gown, wrestling with his dog, Jack, and standing in his underpants in his office as he hurriedly tries to change into a tuxedo.

The documentary captures an acidic confrontation between Mandelson and Osborne, who is now the chancellor, and Labour's frenzied attempt to manage the fallout from Brown's unguarded comments about the Rochdale pensioner Gillian Duffy.

Several Downing Street aides and most of the cabinet make what seem to be unwitting appearances in the documentary, but only one senior Labour figure – Alastair Campbell – questions exactly what Rothschild was doing shadowing Mandelson with a camera.

Rothschild, who makes films about music and art, wrote to Mandelson in October last year asking if he would be the subject of an observational film. Although she had met Mandelson several times socially through her brother she was surprised when he told her to begin filming immediately.

"I would never have given such access to a political or current affairs documentary maker," Mandelson said last night. "I would not have trusted them to do it without their own spin or prejudice. Hannah came to it with an open mind."

He added: "I was busy all the time or in a rush or under pressure and after a while I forgot she was there."

Whether by accident or design, Mandelson allowed Rothschild to accumulate over 200 hours of revealing footage from the election campaign. Her film, Mandelson – The Real PM?, premiers at the London film festival this month and will be broadcast by the BBC in November.

The film, exclusive clips of which are being broadcast by the Guardian, is likely to ruffle feathers in the Westminster establishment.

The documentary opens with the then-business secretary in his ministerial car, speaking to an unidentified official over the phone. "How is the eye?" he asks, in an apparent reference to Brown's faulty eyesight. "Is he worried about going out with it?"

Mandelson is later shown speaking to the prime minister over the phone, reassuring him about his performance in the first televised debate. "There are certain adjustments you can make – tone, style or whatever – and all those can be programmed in," he tells the then prime minister.

Another scene likely to irk Brown reveals concern among his advisers about his unpopularity with the electorate. In one strategy meeting at which Brown was not present, David Muir, his director of political strategy, says the public sees the prime minister as "beaten up and lacking in confidence".

"We can't construct a campaign which on paper gives him definition – as a brand – if it actually saps his energy and saps his confidence," he said.

Mandelson replied that the public "don't like him terribly" and "certainly wouldn't like to go down the pub with him".

"He can have his braininess – he can be as brainy as he likes," Mandelson adds, "but what I also want him to be is sufficiently relaxed and approachable that people can conclude that he is a human being."

During another meeting – less than a month before election day – the senior Downing Street adviser Patrick Diamond confides in Mandelson that the party's manifesto spending plans "don't add up".

"Personally I think we are in a real mess over social care," Diamond said. "We've now discovered that we need £1.5bn to fund the next parliament, but we've discovered since a £3.8bn black hole in the funding for maintaining the current level of social care provision."

The film repeatedly shows Mandelson's famed ability at managing journalists, including a bruising encounter with Harding. Angry over a critical editorial in the newspaper, Mandelson calls the Times editor and rebukes him over the phone.

"I mean I thought it would be a brilliant idea if you actually heard what my views are, and my policies and my ideas, from me, before slamming me for what I don't think, and I'm not doing, for motives I don't possess," he told the editor.

Perhaps the most revealing insight into the modus operandi of the so-called Prince of Darkness comes from his encounters with Osborne, at that time shadow chancellor. The pair have had a fraught relationship since their 2008 encounter in Corfu.

In the film Mandelson is shown theatrically mocking Osborne in front of journalists in the "spin room" during the first televised debate.

Spotting the shadow chancellor in private conversation with the Daily Telegraph's deputy editor, Benedict Brogan, Mandelson shouts across the room: "Is he the only person you have to talk to, George? Come on – this isn't fair!"

In a more private moment after the third debate, the two men are seen sitting together waiting to be interviewed by the BBC. Osborne sarcastically asks: "When are we going to see your film, Peter? June? July?"

Speaking in almost a whisper, Mandelson replies: "I have decided to extend it. We're taking it to Corfu."

• This article was amended after publication to remove an inaccuracy that was introduced during the editing process