A panel of UN experts have determined that the detention of former Pakistani prime minister Imran Khan was arbitrary and in violation of international law, calling for him to be released "immediately".

In an opinion published Monday, the United Nations Working Group on Arbitrary Detention raised serious concerns around multiple cases brought against Khan since he was ousted in April 2022.

It found that his deprevation of liberty violated a string of international laws and norms, and was "arbitrary".

Khan's "detention had no legal basis and appears to have been intended to disqualify him from running for political office", the working group concluded.

"Thus, from the outset, that prosecution was not grounded in law and was reportedly instrumentalised for a political purpose," it said in the opinion, which was dated on March 25 but only made public on Monday.

The working group, made up of five independent experts, whose opinions are not binding but carry reputational weight, called on Pakistan's government to "take the steps necessary to remedy the situation of Mr. Khan without delay and bring it into conformity with the relevant international norms".

"The appropriate remedy would be to release Mr. Khan immediately and accord him an enforceable right to compensation and other reparations, in accordance with international law," they added.

Khan, who served as prime minister from 2018 to 2022, has been entangled in more than 200 legal cases since he was ousted, in what he says is a campaign to keep him from power.

He has been detained since August last year and barred from standing for office.

However, the former international cricket star and his wife had their 14-year prison sentences for graft suspended by a Pakistan high court in April.

Khan then had a 10-year sentence for treason overturned this month but remains in Adiala jail, south of the capital Islamabad, over an illegal marriage conviction.

He had been cleared for release before that trio of sentences in the days running up to Pakistan's February 8 general elections.

Neither Pakistan's interior nor information ministry immediately responded to a request for comment.

Khan's Pakistan Tehreek-e-Insaf (PTI) party meanwhile hailed the ruling as "a huge victory".

"It has shown without any one percent of doubt that Imran Khan is innocent and has been thrown into prison illegally," PTI spokesman Syad Zulfiqar Bukhari said in a statement.