The odds of Machado winning the prize jumped from 3.75 percent to nearly 73 percent overnight Thursday to Friday on the predictive betting platform Polymarket.

But no expert or media outlet had mentioned her being among the favourites for the prize, which was announced just a few hours later in Oslo.

"You don't normally see this in the betting market. It's very suspicious," Robert Naess, a data specialist, was quoted as saying by Norwegian broadcaster NRK.

"I don't think there have ever been any leaks in the entire history of the prize. I can't imagine that's the case," Norwegian Nobel Committee Chairman Jorgen Watne Frydnes told the NTB news agency.

The institute will nevertheless investigate whether there could have been any leaks, its director, Kristian Berg Harpviken, said.

"It's too early to be categorical about the existence of a leak. But it's something we will now look into," he told the Aftenposten newspaper.

The Nobel Institute did not respond to AFP's requests for comment.

An extremely limited number of people know in advance the name of the laureate chosen by the five members of the Nobel Committee.

In the past, however, unexpected names of Nobel nominees have emerged in the Norwegian media, fuelling speculation about possible leaks, but this has not been the case in recent years.

Machado, an opposition leader barred from running in Venezuela's 2024 presidential election, was awarded "for her tireless work promoting democratic rights for the people of Venezuela and for her struggle to achieve a just and peaceful transition from dictatorship to democracy", the committee said.