Charles Sobhraj, a convicted killer who police say is responsible for a string of murders in the 1970s and 1980s, was released from a Nepal prison on Friday after nearly 20 years behind bars, according to a Reuters witness.

Dubbed the "bikini killer" in Thailand, and "The Serpent", for his evasion of police and use of disguises, Sobhraj, 78, a French national, is suspected of killing more than 20 western backpackers on the "hippie trail" through Asia.

His notoriety and exploits have been the subject of several dramatisations, including a Netflix and BBC joint production released last year.

Sobhraj was expected to be taken from jail to the immigration department in the capital Kathmandu to complete his paperwork and enable him to return to France.

Nepal's Supreme Court on Wednesday ordered his release from prison, citing his age.

He was expected to be out of jail on Thursday, but pre-release procedures, including a health check-up, have caused delays, Ishwari Prasad Pandey, a warder at the Central Jail in Kathmandu told Reuters late on Thursday.

Sobhraj has been held in a high-security prison in Nepal since 2003, when he was arrested on charges of murdering American backpacker Connie Jo Bronzich in 1975. 

He has served 19 years of a 20-year sentence.

"I'm happy and have great respect for our judiciary and Supreme Court," Sobhraj's mother-in-law Sakuntala Thapa told Reuters partner ANI after news of his release was announced.

Escape from Indian jail after drugging guards

Sobhraj married Nihita Biswas, a Nepali woman 44 years his junior, in 2008.

He denied killing Connie Jo Bronzich, and his lawyers said the charge against him was based on assumption.

Several years later, Sobhraj was also found guilty of killing Ms Bronzich's Canadian friend, Laurent Carriere.

But he was suspected of many more murders, including in Thailand, where police say he allegedly drugged and killed six women in the 1970s, some of whom turned up dead on a beach near the resort of Pattaya.