Caitlyn Jenner and Harry Redknapp have both accepted thousands of pounds in return for backing a fake charity set up by Channel 4 in a sting operation.

In a Dispatches documentary, Celebs For Sale: The Great Charity Scandal, to be broadcast on Monday evening, the two well-known faces are revealed to be part of a widespread practice of paying celebrities for public support.

By inventing a charity – called Cleaning Up Plastic Pollution in Africa, or Cuppa – journalists were able to prove that financial transactions took place.

“When we started this investigation, I never imagined I would end up in Caitlyn Jenner’s Malibu house discussing her support for the charity we had created,” the documentary’s presenter, Antony Barnett, said this weekend. “To keep public trust in the charity sector, it is important they are all open and transparent about where their money is going.”

Barnett added that he had talked to other celebrities who had expressed concern about the practice, “but none was willing to speak out”.

Celebrity booking agents told Dispatches that Redknapp would back Cuppa at a cost of £15,000 for one Instagram post, with an extra £5,000 for a photoshoot. A contract was signed and the programme makers paid the agency £20,000. A week later a photograph of Redknapp wearing a Cuppa T-shirt and cap and holding a themed cup was posted on his Instagram account.

“I’m proud to support Cuppa – a great new charity that’s Clearing Up Plastic Pollution in Africa. Follow them on insta to show your support,” Redknapp wrote.

Harry Redknapp’s lawyers said he would ‘never knowingly charge for charity work’. Photograph: Chris Brunskill/Fantasista/Getty Images

A lawyer for Redknapp said: “Mr Redknapp is a prolific supporter of charity and has raised considerable sums over the years. He receives no fees for such support and would never knowingly charge for the regular charity work he undertakes … In this case he only agreed to payment after being informed that the charity was supported by private investors... He was paid less than the sum quoted by MN2S [the talent agency].”

The programme makers also paid MN2S more than £19,500 for Jenner to take part in a photoshoot and share an Instagram post. The star then talked about her support for Cuppa in a video made on her phone, saying: “I think we have to be good stewards of this planet. And the plastic issue is huge.”

The next day she posted a selfie in which she wears a Cuppa T-shirt and holds a Cuppa cup, but did not make clear she had been paid.

A lawyer for Jenner said: “It is well- known to the public that sometimes celebrities are paid fees for doing promotional work for charities. Apart from this instance, she has never been paid for her charitable work, and always intended to donate the money raised via the photoshoot and Instagram post to charities she supports ... She feels that she has been targeted in the programme because of her status as a well-known TV personality and that she has been exploited to increase viewing figures.”

MN2S said: “Many of the celebrities that MN2S work with undertake charity work for free. In addition, they are regularly approached to undertake one-off fundraising work – as was the case with your fake charity Cuppa. In those instances, we may agree a fee that covers not just the individual but the work of the team that make it possible. The fee is often heavily discounted from the commercial rate, as was the case with the celebrities we work with and Cuppa.”

A spokesperson for Harry Redknapp said: “As everyone knows, Harry does a massive amount of work for charities big and small, and has done for many years, which he doesn’t charge for.

“He is disappointed that he was deceived into believing that private investors wanted to engage his services, and would not have accepted payment if he had been told this was being paid for by the charity in question. Harry’s 2019 theatre tours raised over £160,000 for charity, and he is doing the same again this year. Harry is also either a patron or trustee of 16 different charities. Harry is shocked and upset that he was misled in this way and is donating the money he was paid to the charities he works with so that it can be put to good use.”

• This article was amended on 8 March 2020 to include a further statement from a spokesperson for Harry Redknapp that was received after publication.