China is planning to bail three Hong Kong booksellers being held on the mainland, police said Wednesday, in a case that heightened fears over freedom of speech in the semi-autonomous city.

Hong Kong police said their counterparts in the southeastern city of Guangdong had told them Lui Por, Cheung Chi Ping and Lam Wing Kee would be released on bail "in the coming few days".

Five booksellers from Hong Kong's Mighty Current publishing house, known for its salacious titles critical of Beijing, have disappeared since October only to turn up in mainland China.

Their case has added to fears China is tightening its grip on the city in the wake of 2014 pro-democracy protests and sparked a row between Beijing and former colonial power London.

Hong Kong police said the three men's families have been informed of their bail, but the statement did not make clear if they will be able to return home.

"Police received a reply letter... stating that Lui Por, Cheung Chi Ping and Lam Wing Kee were suspected to be involved in a case relating to a person surnamed Gui," the police statement said.

"They will be released on bail pending investigation in the coming few days."

The three men made their first appearance since being detained on Sunday, when they confessed to illegally smuggling books into the mainland in interview with Hong Kong-based Phoenix TV.

They blamed a fourth bookseller, Swedish national Gui Minhai, who appeared weeping on Chinese state television in January saying he had turned himself in for a fatal driving accident 11 years ago.

Chinese news outlet Paper.cn said Cheung, Lui and Lam may "return to Hong Kong in the near future" on bail pending trial because they "confessed with good attitudes".

There is no news on the fate of a fifth bookseller, Lee Bo, whose case sparked the biggest backlash as he was the only one to disappear from Hong Kong.

Lawmakers and activists have accused Chinese authorities of snatching him, contravening the semi-autonomous city's laws which forbid mainland police from operating within the territory.

Britain said last month it believes Lee was "involuntarily removed to the mainland" in what it called a "serious breach" of an agreement signed with Beijing before the city was handed back to China in 1997.

Beijing said the accusations were "groundless" and told London to "mind its words and actions and stop interfering in Hong Kong affairs".