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 as being among the favourites for the prize, which was announced just a few hours later in Oslo.

"Highly likely it's espionage," the director of the Nobel Institute and secretary of the Nobel committee, Kristian Berg Harpviken, told Norway's TV2 television.

On Friday, the head of the Nobel committee said he didn't believe Machado's name had been leaked.

"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," committee chairman Jorgen Watne Frydnes told the NTB news agency.

Harpviken said the institute would nevertheless investigate and "where necessary, we will further tighten security".

Espionage "could make it appear as if someone on the inside deliberately leaked information. That is not likely," he said.

"It's too certain to say for sure, but it's no secret that the Nobel Institute is subject to espionage," he added.

"It is obvious that the institution is of interest to actors who want to acquire information, both states and other organisations," he said, adding: "The motives can be both political and economic."

"This has been going on for many decades."

Harpviken 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, 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.

The Nobel Peace Prize has long been coveted by US President Donald Trump, whose office called the committee's decision to award it to Machado instead of him a sign of "politics over peace".