Colombian President Juan Manuel Santos and FARC chief Rodrigo Londono, alias Timoleon "Timochenko" Jimenez, had until now appeared to be serious contenders for the prestigious award after signing a deal on September 26th to end 52 years of civil war.
But when the historic deal was put to a referendum on Sunday, voters in Colombia rejected it by a razor-thin majority, throwing the country's future into question -- and forcing Nobel observers to rethink their predictions ahead of the October 7th announcement.
"In this context ... the Colombian peace treaty or anybody associated with it simply is not a candidate for the Nobel Peace Prize this year," said Kristian Berg Harpviken, director of Oslo's Peace Research Institute (PRIO).
"I think it is simply off any credible list," he added, noting that any such award would be perceived as flying in the face of the will of the Colombian people.
Nobel Peace Prize historian Asle Sveen, who had also seen the architects of the Colombia accord as the favourites, agreed.
"It's highly unlikely," he said, even though the Norwegian Nobel committee has in the past awarded the prize to those involved in peace efforts which had yet to bear fruit, with the express aim of encouraging them.
"Yesterday's referendum result is a terrible blow to peace prospects in Colombia," said Dan Smith, head of the Stockholm International Peace Research Institute (SIPRI) who had also been betting on the Colombians.
"It makes the Nobel award much less likely but I would not completely rule it out," he told AFP.
In the event that the Nobel committee had decided to award the prize to the Colombians, it must now rush to find a last-minute replacement.
Experts said that given the very uncertain nature of a referendum, it was likely that the committee had a plan B in place. But it's anyone's guess who that might be.
Among those listed as possible contenders are Russian rights activist Svetlana Gannushkina, the negotiators of the Iranian nuclear deal Ernest Moniz of the US and Ali Akbar Salehi of Iran, and Syria's White Helmets volunteer rescue force.
US fugitive whistleblower Edward Snowden has also been touted as a possible winner, as has France's former foreign minister Laurent Fabius for his role as head of the COP21 Paris climate accord.
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 -- with the winner to be announced on Friday at 11am in Oslo.