The body said that "senior Belarusian officials" had sent a "deliberately false" bomb threat to aircraft to divert it to Minsk and arrest a dissident journalist.
The UN's aviation agency has formally blamed Belarus for diverting a Ryanair flight last year to arrest a dissident journalist.
The Belarusian government committed an "act of unlawful interference," the International Civil Aviation Organization (ICAO) said after completing its investigation.
Ryanair flight FR4978 was forced to land in Minsk on 23 May 2021 while on its way from Greece to Lithuania. Belarusian authorities claimed they had grounded the aircraft because of a bomb threat on board.
Upon landing in Minsk, officials arrested two passengers -- Belarussian journalist blogger and political activist Roman Protassevich and his Russian girlfriend Sofia Sapega.
On Wednesday the ICAO declared that the bomb threat was "deliberately false" and had endangered the flight's safety.
It added that the hoax message "was communicated to the crew on the instructions of senior Belarusian officials".
The ICAO cannot itself impose sanctions but can advise countries when there is a proven violation of international aviation rules. The European Union and other Western nations imposed sanctions on Belarus after the Ryanair flight was re-routed.
The organisation's Russian representative has disputed that "Belarus was identified as the source of this illegal interference," the ICAO added.
Last month, 23-year-old Sapega asked to be pardoned by Belarusian President Alexander Lukashenko after being jailed for six years for "inciting social hatred" and "illegal collection of personal data".
Protasevich, 26, who co-founded the opposition Telegram news channel Nexta, reportedly remains under house arrest in Belarus, pending trial.
He faces charges that include organising mass unrest, which could land him with up to 15 years in prison.
Since their capture, Protasevich and Sapega have been paraded on Belarusian TV and shown confessing to criminal activity in recordings which appeared to have been made under duress.
His family and supporters believe he was coerced into releasing videos denouncing his political activism.
Belarus has been ruled by the hardline Lukashenko since 1994. The staunch Putin ally withstood months of mass demonstrations in 2020 as opposition politicians and activists were arrested and imprisoned.