Gui, one of five Hong Kong-based booksellers known for publishing salacious titles about China's political leaders, was sentenced to 10 years in prison in 2020 on espionage charges.

Human rights experts called Gui's detention "arbitrary" in an opinion adopted by the United Nations Working Group on Arbitrary Detention (WGAD) dated October and made public on Wednesday.

Asked about the WGAD call for the bookseller's release, Beijing's foreign ministry spokesman Guo Jiakun said Gui was "sentenced by a Chinese court for serious criminal offences".

"China is a country governed by the rule of law, where judicial authorities strictly handle cases in accordance with the law," the spokesman told reporters at a regular briefing.

Beijing "opposes any interference in its judicial sovereignty by any country, organisation or individual in any form", he added.

WGAD said that "taking into account all the circumstances of the case, the appropriate remedy would be to release Mr Gui immediately and accord him an enforceable right to compensation and other reparations, in accordance with international law".

The group is made up of five independent experts appointed by the UN Human Rights Council and investigates "cases of detention alleged to be imposed arbitrarily or otherwise inconsistently with international human rights standards", according to a WGAD fact sheet.

Gui disappeared while on holiday in Thailand in October 2015 and resurfaced in China, where he served two years in prison.

A few months after his October 2017 release, he was again arrested -- this time while on a train to Beijing with Swedish diplomats -- and was later hit with the 10-year jail term.

Gui was born in China but moved to Scandinavia following the Tiananmen crackdown in 1989, and later became a Swedish citizen.

China does not recognise dual citizenship, and Chinese officials claimed he voluntarily reinstated his Chinese citizenship in 2018.

Sweden insists he remains a citizen and has repeatedly called for his release.