Caitlyn Jenner and Harry Redknapp both accepted large fees to back a charity set up by undercover reporters.
An operation by Channel 4's Dispatches programme-makers found famous faces would take tens of thousands of pounds to show public support for a charity reporters had invented.
The programme invented the dummy charity Cleaning Up Plastic Pollution in Africa (CUPPA), built a basic website and created some merchandise, but did not register it as a charity and did not seek donations.
After contacting celebrity booking agents, Dispatches was told Redknapp would be willing to back the charity but it would cost £15,000 for him to do one post on Instagram and an extra £5,000 for a photoshoot.
The programme signed a contract, paid the agency £20,000 and secretly filmed the photoshoot.
One week later, a photograph of Redknapp wearing a CUPPA T-shirt and cap and holding a themed cup was posted on his Instagram page.
He wrote: "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."
He did not disclose that he was paid for the post.
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... His support is of enormous financial benefit to these charities.
"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 also paid MN2S more than £19,500 for Jenner to take part in a photoshoot and share an Instagram post.
In a video filmed on a phone, Jenner speaks about why she is backing the charity, saying: "I think we have to be good stewards of this planet. And the plastic issue is huge.
"And, you know, not that it's easy to do, but it can be done, is to clean the beaches up, clean areas up, especially plastics because they don't go away. And we need to be good stewards. And it is something that we can fix."
Jenner posted a photograph of herself wearing a CUPPA T-shirt and holding a CUPPA cup on her Instagram the next day, but did not make clear she was 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.
"The benefit is mutual... 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."
A representative for 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.
"This is widely known within the charity sector. Charities book celebrities because it raises their profile and helps them often raise far more money than the amount paid to the celebrity. The fee is often heavily discounted from the commercial rate, as was the case with the celebrities we work with and CUPPA."
Another talent agency approached in the programme, Champions plc, said: "The vast majority of our charity engagements are free of charge.
"Over the last three years, Champions has given more than 10% of its annual profits to charity.
"When approached undercover by Dispatches, we believed that the brief was to help raise the profile of an unknown charity that wished to engage celebrities to perform at a private launch event funded by private benefactors.
"It would be wrong to conclude that all those named would have been aware that they were even under discussion.
"Champions are a responsible organisation and will of course watch the programme carefully. Should it raise any concerns we will investigate and take action where appropriate."
Celebs For Sale: The Great Charity Scandal is on Dispatches on Channel 4 at 8pm on Monday.