Adventurer Emma Kelty, 43, from London, was last heard from on Wednesday September 13, days after posting about her fears of being robbed or murdered.
The former headteacher’s last known location was around 150 miles west of the jungle city of Manaus, between the towns of Coari and Codajas.
Ms Kelty’s body has not yet been found but a teenage boy has said he was involved in her murder along with six other people, according to Brazilian police.
A Foreign office spokesman said on Tuesday: “We are supporting the family of a British woman following her death in Brazil and are in contact with the Brazilian authorities.”
The kayaker disappeared along the upper stretches of the Amazon, often referred to as the Solimoes river.
The area, in northern Brazil, is known for drug traffickers and pirate attacks.
Police discovered a kayak identical to the one in Ms Kelty’s social media pictures close to the river.
Officers also said they found criminals trying to sell some of the kayaker’s belongings in a local community.
Ms Kelty had been attempting a 4,000-mile trip along the Amazon river from its source to the Atlantic Ocean.
She had also been documenting her trip on Twitter, with her last tweet coming on 13 September.
Just the day before, Ms Kelty tweeted she saw “30 to 50 men armed with rifles and arrows in boats”.