A veteran Russian rights activist, or the architects of the Iranian nuclear accord? The field of contenders for Friday's Nobel Peace Prize was suddenly thrown wide open after the Colombian people's shock rejection of a peace deal put its negotiators out of the running.

For once, the Nobel experts thought they were on to a sure thing.

But just days before the award, they were forced to rethink after voters in Colombia said 'No' to a peace deal between their government and the communist FARC rebels in a weekend referendum.

Until now, Colombian President Juan Manuel Santos and FARC chief Rodrigo London, alias Timoleon "Timochenko" Jimenez, had been tipped as favourites to win the prestigious award after signing a deal on September 26 to end 52 years of civil war.

In the event that the Nobel committee had decided on Colombia, then it would likely have had a plan B in place, given the unpredictable nature of referendums, experts said.

But it remains to be seen who that might be.

For some, the prestigious award could go to the negotiators behind the 2015 Iranian nuclear accord which effectively curbed Tehran's nuclear drive, putting an atomic bomb out of reach, in exchange for a gradual lifting of the crippling sanctions imposed on its economy since 2006.

"As the agreement now has demonstrated that it works, this might now be a strong contender," said Peter Wallensteen, a professor at Sweden's Uppsala University.

That could see the prize going to Washington's top diplomat John Kerry, his Iranian counterpart Javad Zarif and EU foreign policy chief Federica Mogherini, as well as to nuclear experts Ernest Moniz, the US Energy Secretary, and Ali Akbar Salehi, head of Iran's Atomic Energy Organization.

In line with the wishes of prize creator Alfred Nobel, the Swedish inventor who wanted to reward disarmament efforts, such a prize could serve to consolidate the nuclear deal ahead of the possible arrival of a new US president who may be less well-disposed towards Iran, said Kristian Berg Harpviken, director of Oslo's Peace Research Institute (PRIO).

- Record number of candidates -

Harpviken believes the top contender is Russian human rights activist Svetlana Gannushkina for her decades-long work with migrants and refugees, an issue which has shot to prominence in Europe since the start of the migrant crisis last year.

"Since the decline of open democracy in Russia started, particularly with the re-entry of President (Vladimir) Putin into the presidency, there has been no Nobel Peace Prize casting a light on developments in Russia," Harpviken said, suggesting that such an omission could one day reflect badly on the Norwegian institution.

Among others listed as possible contenders are Syria's White Helmets volunteer rescue force, Greek islanders for their efforts to help desperate migrants landing on their shores, Congolese doctor Denis Mukwege for his work with thousands of rape victims, and Nadia Murad, a Yazidi who was abducted by IS fighters and held for months as a sex slave.

Whistleblower Edward Snowden, who exposed the scope of US surveillance, has also been touted as a possible winner, as has France's former foreign minister Laurent Fabius for his role as head of COP21 Paris climate accord.

- Trump in the running? -

This year, the Norwegian Nobel Institute has received a whopping 376 nominations for the peace prize, a huge increase from the previous record of 278 in 2014 -- meaning the number of choices facing the Nobel committee is vast.

But if they're struggling to agree on who might be the winner, experts say there is one name who they all agree won't be getting it -- US Republican presidential candidate Donald Trump, who was nominated for his "vigorous peace-through-strength ideology".

In an illustration of just how difficult it is to call, last year's prize went to four Tunisian groups who were instrumental in the country's transition to democracy. They had not been mentioned in any of the pre-announcement speculation.

"Perhaps they will have a new rabbit to pull out of their hat again on Friday," remarked Nobel Peace Prize historian Asle Sveen.

The mystery will only be solved on Friday at 0900 GMT when the prize is announced at the Nobel Institute in the Norwegian capital, Oslo.