A candid documentary following Peter Mandelson in Labour’s last days in power is to be screened next month.

‘Mandelson – The Real PM?’, filmed by Hannah Rothschild, shows the former business secretary in the eight months up to the May election.

According to the Guardian, the documentary is remarkably candid, showing Lord Mandelson having sensitive conservations and talking about Gordon Brown’s unpopularity.

In other scenes, he is shown wrestling his dog, in his dressing gown at home and standing in his office in his underpants while getting changed.

Lord Mandelson is also shown mocking chancellor George Osbourne, rebuking a newspaper editor and telling a meeting that the electorate dislike Mr Brown.

Ms Rothschild, who is the sister of financier Nathaniel Rothschild and has met Lord Mandelson socially, said she was surprised to have been given such free access to the man seen as the de facto deputy prime minister.

She told the Evening Standard: “He said he wanted a record of what politics is really like. And that’s what I’ve done – warts and all.”

Lord Mandelson told the Guardian last night that he eventually forgot that Ms Rothschild was there and that she had come to the film with an “open mind”.

The film will premier at the London film festival this month and will be broadcast by the BBC in November.

Click here to watch clips from the film