Fears are growing for a missing British adventurer who may have been snatched by pirates or drug runners while canoeing in the Amazon.

The day before she vanished Londoner Emma Kelty posted a message on Facebook about running into "50 guys in motor boats with arrows!!!" in the rainforest.

Brazilian navy patrol boats and helicopters are searching for the 43-year-old, an experienced canoeist who was attempting a 4,000 mile solo journey from Peru to Brazil.

The alarm was raised after her boat and belongings were found on the sandbanks of the Solimoes River in the northern jungle state of Amazonas.

This stretch of river is part of a drugs trafficking route and known for frequent pirate attacks, and is one of the most dangerous parts of her trip.

The teacher is said to have fired an emergency locator last Wednesday evening (13 September) at 10pm, Brazil's 9th naval district command said.

The navy, who are working with authorities, believe the woman was between the cities of Codajas and Coari when she signalled for help.

Ivo Martins, the state's civil police chief, said: "There are various lines of inquiry. She could have been killed by drugs traffickers, or attacked by pirates, but she could also have been attacked by an animal, or just got lost".

Forensics experts are analysing her belongings including cigarette butts, food, clothes and safety equipment recovered from her canoe, which was found abandoned on a sandbank on Friday (15 September).

On Monday (18 September) the Brazilian Navy said a 60-strong search party were still looking for the woman, who they described as a "professional sportswoman who has experience in navigation".

"The information that we have is that her objective was to follow the Amazon river all the way to its mouth," said Paulo Veiga, the captain of the district naval command in Manaus.

Police chief Martins added that the search efforts will continue until "all possibilities of finding the English woman have been exhausted".

She passed through the Brazilian border post with Colombia at Tabatinga, 500 miles from where she went missing, on 21 August.

Kelty had been documenting her journey on her blog and social media and using a GPS tracker so friends and family could follow her.

A Foreign Office spokeswoman said: "We are supporting the family of a British woman who has been reported missing in Brazil, and are in contact with the Brazilian navy."

The state's previous civil police chief, Thyago Garcez, disappeared in December in almost exactly the same remote stretch of the Solimoes river after an incident involving drug traffickers. His body has never been found.

Just last month a boat navigated by drugs traffickers from Peru was attacked by pirates who robbed the cargo after a fierce gunfight in the same area, according to reports.

Kelty, whose family are understood to live in Taunton, Somerset, had been working at the Knollmead Primary School in Surbiton, south-west London, but quit to go travelling.

The adventurer became the sixth woman to ski unassisted to the South Pole in January this year, and has also completed a 2,600-mile solo hike across the US among other challenges.