Sir Paul McCartney, Peter Capaldi and Joanna Lumley are backing a campaign to prevent "destructive" hydroelectric dams being built in the Amazon.
Dozens of celebrities have signed an open letter urging the Brazilian government to recognise the land rights of the indigenous Munduruku people, who are fighting against a series of dams which threaten their territory.
Plans for one of five dams across the Tapajos River were turned down by the Brazilian environment agency due to the unacceptable damage it would cause in the Amazon and the Munduruku lands.
But more hydroelectric dams are still planned across the river, the last major undammed tributary of the Amazon, including some that would flood Munduruku territory, campaigners warn.
In an open letter published in the Guardian, actors Sir Roger Moore, Mark Rylance and Olivia Colman, singer Charlotte Church and presenter Dermot O'Leary were among those calling for the Munduruku's rights to be recognised and for dam construction companies to "stay out of this destructive business".
They warned: "The Tapajos river and the surrounding rainforest are areas of unparalleled natural beauty and biodiversity, where new animal species are still being discovered to this day.
"The Brazilian watchdog's decision marked a turning point in the struggle to protect this corner of the Amazon, but the fight isn't over yet."
Members of the indigenous Munduruku people have come to the UK seeking a meeting with the bosses of Siemens UK, as the company is part of the consortia that built Brazil's last four mega dams and is one of the few companies that supplies the components of large-scale hydroelectric projects.
General chief Arnaldo Kaba Munduruku and his senior adviser Ademir Kaba Munduruku, have come to the UK from the Amazon to ask Siemens to publicly pledge not to participate in plans for new dams on their ancestral lands.
Arnaldo Kaba Munduruku, said: "The river and the forest give us everything we need to survive. They give us our food, our water, our drugs. If they build this dam they will kill the river, they will kill my people, our culture.
The future of our children is threatened by the ambition of the companies and the government. The forest is also important for other people from other countries because it belongs to everybody."
Greenpeace, which is working with the Munduruku's campaign to save the Tapajos, said more than a million people from around the world had signed up to support the fight, and called on Siemens to listen to those people and state they would not be involved in the dams.