* Colombia had been favourite for prize before Sunday's vote
* Russian rights campaigner, Iran among tips for prize
By Alister Doyle and Terje Solsvik
OSLO, Oct 3 (Reuters) - Peace researchers dropped Colombia on Monday from a list of favourites for the Nobel Peace Prize after Colombians voted "No" in a referendum to an agreement to end a 52-year war with Marxist rebels.
Sunday's surprise rejection of the accord, after criticism that it was too lenient to the rebels, improved chances for other Nobel candidates such as Russian human rights activists or brokers of Iran's nuclear deal to take the peace award, they said.
"Colombia's off any credible list," Kristian Berg Harpviken, head of the Peace Research Institute, Oslo, speaking to reporters about the 2016 Nobel Peace Prize that will be announced in Oslo on Friday at 0900 GMT.
President Juan Manuel Santos and FARC's top commander Rodrigo Londono, better known by his nom de guerre Timochenko, had been widely tipped for the 8.0 million Swedish crown ($936,000) award before the referendum.
The prize has often gone to encouraging peace processes, such as in Northern Ireland in 1998, between Israelis and Palestinians in 1994 or even in Vietnam in 1973, but never in defiance of a popular vote.
"It's now out of the question" to give a prize for Colombia, said Asle Sveen, a historian who tracks the prize. He had previously tipped the Colombian agreement to win, for ending a war in which more than 220,000 people died.
Sveen said he now thought the award would go to the agreement between Iran and world powers to end sanctions on Tehran in return for shrinking its nuclear programme.
Possible candidates included U.S. Secretary of State John Kerry, Iranian Foreign Minister Mohammed Javad Zarif and European Union foreign policy chief Frederica Mogherini, he said.
Harpviken, who had Colombia second on a list distilled from 376 nominees, reaffirmed his favourite as Svetlana Gannushkina, a Russian human rights campaigner who focuses on refugees and migrants.
Thousands of people, including members of all national parliaments, professors of international relations and former winners, can make nominations for the award.
Norwegian Foreign Minister Boerge Brende, whose country helped broker the Colombian agreement, expressed disappointment at the vote. "We have to try to rescue the peace agreement," he told independent TV2.
($1 = 8.5441 Swedish crowns)
Our Standards: The Thomson Reuters Trust Principles.