Dom Phillips, a freelancer who has written about Brazil for The Guardian, The Washington Post, The New York Times and others, went missing with Bruno Araujo Pereira, a local expert and former government official whose job was to protect Brazil's uncontacted tribes.

The pair were last seen over the weekend in Javari Valley of Amazonas state - near the border with Peru. The Javari region is home to the most uncontacted indigenous people in the world.

His sister Sian Phillips told Sky News she is concerned there is illegal logging and drug trafficking in the area where he disappeared.

According to The Guardian, Mr Pereira had received a number of threats from loggers and miners in the region.

"I'm very anxious. I'm desperately worried. It's your worst fear," Ms Phillips said.

"We need everything thrown at this. We want UK officials to put pressure on the Brazilian authorities to act."

She described him as a "bright star", adding: "He's my elder brother, I look up to him, I love him.

"I just think my brother is highlighting the situation there at this time and we need to make the most of this situation by saying that."

Ms Phillips said that local indigenous community raised the alarm on Sunday afternoon, when her brother and Mr Pereira failed to return from a three-hour trip.

'We knew it was a dangerous place'

In a separate emotional video appeal posted on social media, she fought back tears as she called for urgency from the authorities.

"My brother Dom has been living in Brazil with his Brazilian wife," she said. "He loves the country and cares deeply about the Amazon and the people there.

"We knew it was a dangerous place but Dom really believed it's possible to safeguard the nature and the livelihood of the indigenous people.

"He is a talented journalist and was researching a book when he disappeared yesterday."

She continued: "We are really worried about him and urge the authorities in Brazil to do all they can, search the route he was following.

"If anyone can help scale up resources available for the search, that would be great, because time is crucial. Every minute counts."

The UK's acting ambassador in Brazil, Melanie Hopkins, wrote on Twitter: "We are following the case of the disappearance of the British journalist Dom Phillips, who was with the indigenous Bruno Araújo Pereira in the Amazon.

"We are in contact with local authorities and offering consular support to the family. Hope we have good news soon."

Vicky Ford MP, who is the Minister for Africa, Latin America and the Caribbean, added that the UK embassy in Brazil is in touch with local authorities in Brazil and "are also offering consular support to his family".

Brazil's Navy on Monday dispatched a crew of 10 people to search for Mr Phillips and Mr Pereira. Brazil's federal police said they are also working to locate the pair.

A spokesperson for Britain's Foreign Office said it is in contact with Brazilian authorities.