A Swedish bookseller whose mysterious disappearance has sparked fears he may have been taken by Chinese agents said he had voluntarily turned himself in to the authorities for a drink-driving accident that resulted in a death 11 years ago.

Gui Minhai, who vanished from his apartment in Thailand last October, voluntarily returned to China to answer a conviction from 2004 for killing a student, state media said on Sunday.

"I am returning to surrender by personal choice, it has nothing to do with anyone," Gui, looking distraught, said in a China Central Television (CCTV) broadcast. "This is a personal responsibility that I ought to bear."

Gui, a naturalized Swedish citizen, is one of five members to have gone missing from a Hong Kong bookstore that specialises in selling gossipy political books on China's ruling Communist Party leaders.

The disappearances, and China's silence, have prompted fears that mainland Chinese authorities may be using shadowy tactics that erode the "one country, two systems" formula under which Hong Kong has been governed since its return to China from British rule in 1997.

In recent years, state media has publicised a string of what is presented as confessions made by high-profile suspects. Critics say these accounts deprive the accused of the right to a fair trial.

Earlier this month, the Swedish Foreign Ministry said it had raised Gui's case with the Chinese ambassador to Stockholm.

Gui's daughter Angela, who is based in Britain, told Reuters late on Sunday she believed the Swedish authorities were doing everything they could to help with the case.

It was not possible to contact Gui and it remains unclear whether he has a lawyer.