A six-week expedition starting in July will try to find Paititi in the Megantoni National Sanctuary in south-east Peru

A French writer and adventurer plans to explore one of the most remote parts of the Peruvian Amazon in search of a "lost" or "secret city" that may have been built by the Incas, but there are fears that the expedition could endanger the health of isolated tribes that have never been exposed to common human diseases.

Thierry Jamin believes that the city, which he calls "Paititi", could lie somewhere in a 215,000-hectare protected area called the Megantoni National Sanctuary in the Cuzco region of south-east Peru.

"The magnificent discoveries realised by my group in the valleys of Lacco, Chunchusmayo and Cusirini in the north of the department of Cuzco lead towards a precise zone situated in the national sanctuary of Megantoni," Jamin told the Guardian via email.

"Several natives of the forest – Matsiguengas – assert that 'monumental ruins' exist at the top of a strange square mountain. I think that we are very close to officialise the existence of this big archaeological site."

According to his website, Jamin is planning a six-week expedition starting in July. He will be assisted by an NGO based in Cuzco that he leads and a group of Machiguengas from a village near the sanctuary.

His website describes Paititi, or "Paititi-Eldorado", as the "Incas' secret city" – "one of the most fascinating stories of the Inca mythology", the "biggest archaeological enigma of South America", and the place where the Incas hid "all the treasures of [their] empire" when Europeans invaded.

The search for Paititi or an Inca "lost city" has attracted scores of people and considerable controversy ever since the 16th century, with conflicting theories and ideas about where it might be and whether it really exists.

But some experts fear that such an expedition would pose a threat to isolated indigenous Nanti people – sometimes called "Kugapakoris" – within the sanctuary. One of the main reasons for the sanctuary's creation 10 years ago was to protect groups of indigenous people who have had little or no contact with outsiders and are extremely vulnerable to infectious diseases because of their lack of resistance.

According to the sanctuary's "master plan" for 2007-2011 – a 160-page government document outlining strategies and programmes to manage the area – the 215,000 hectares are divided into a number of "zones" where different activities are permitted. The biggest, most remote zone is in the sanctuary's far east and is called the "Strict Protection Zone" (ZPA). Its first stated aim is to protect "voluntarily isolated indigenous people", with scientific investigation only allowed in "exceptional circumstances".

Jamin is keeping the precise destination of the expedition a secret, but told the Guardian he intended to travel up the river Ticumpinia – not the river Timpia where he said there were "numerous Kuga-Pakuri communities".

"We don't want to tell anyone about our study zones, nor disseminate the exact locations of the sites we have found," he said.

Lelis Rivera, who works for the NGO Cedia and played a key role in the sanctuary's creation, pointed out that the presence of any outsiders in the sanctuary "could cause danger to the people living there" and that entering the ZPA in particular is "completely prohibited" by Peruvian law.

"Any people currently living in the upper Timpia or Ticumpinia regions are extremely vulnerable to germ transmission – that's the nature of living in relative social and immunological isolation," said anthropologist Christine Beier from the NGO Cabeceras Aid Project and one of the world's leading experts on Nanti society and history.

Jamin told the Guardian that he will apply to the ministry of environment, which oversees management of "protected natural areas", for permission to enter the Megantoni sanctuary. He said he has already applied for permission from the ministry of culture.

However, Ramon Rivero Mejia at the culture ministry says it has received no application from either Jamin, any member of his team or the NGO that Jamin presides over.

Some experts doubt that Paititi is where Jamin thinks it is. "The Incas conquered territories of the Machiguenga and Piro and built roads, bridges and some fortified settlements, meaning it's possible that in Megantoni some Inca-type buildings and objects will be found," said Martti Parssinen, a Finnish archaeologist and historian who has researched Peru and the Incas for decades.

"Nevertheless, Paititi is not there … At first, it was located from the confluence of the Madre de Díos and the Beni rivers toward the east or south, but during the colonial period some Inca refugees probably reestablished it near the present Brazilian Pacaas Novos mountains."

Asked by the Guardian why he thinks archaeological remains in Megantoni might be related to the Incas, Jamin said: "We don't know if they're Inca or pre-Inca. One of the objectives of our 2014 campaign will be to establish that."