The wife of one of the five missing Hong Kong booksellers has told police she has been able to visit him at a “secret location” on the mainland.

In a new twist in the saga, the Hong Kong police announced it had been contacted by Choi Ka-ping, the wife of Lee Bo, one of the five missing men, claiming to have visited her husband in an unnamed guesthouse across the border.

The announcement, made at 2am local time, states that Lee’s wife said she told them he “was healthy and in good spirits, and said that he was assisting in an investigation in the capacity of a witness”.

Lee Bo, 65, a British citizen, went missing from Hong Kong on 30 December, and entered the Chinese mainland without using his travel documents. Since then, he has released a number of letters and faxes, claiming to have gone to Hong Kong “by his own means” and asking Hong Kong police to “stop investigating his case”.

However, the border authorities have no record of his leaving Hong Kong and the case that has raised suspicion that Chinese law enforcement officials may have abducted him from Hong Kong territory.

The five missing men work for the Hong Kong-based publishing house Mighty Currents, known for books that are highly critical of the Chinese Communist Party.

The police statement, revealing that the Hong Kong authorities had been kept in the dark about the arrangements for the visit, has sparked fury among local activists.

“This is entirely unacceptable”, said Fernando Cheung, a pro-democracy MP, “up to now the whole handling of the Lee Bo fiasco is being conducted through unofficial means, bypassing the Hong Kong authorities, and Lee Bo seems to be releasing messages under the gun”, he said.

“The British government should speak out strongly about the whole fiasco and demand to meet with Chinese officials and secure the safety of Lee Bo immediately.”

Nicholas Bequelin, East Asia director at Amnesty International, joined calls for the Hong Kong government to stand up to Beijing.

“If the Hong Kong government tolerates the methods of the mainland’s political police, whose hallmark is to operate outside of the law through enforced disappearances, arbitrary detentions, forced confessions and torture, then Hong Kong will cease to be Hong Kong”, said Nicholas Bequelin, East Asia director at Amnesty International.

Hong Kong police said they are continuing to investigate Lee’s case and had again asked police in Guangdong province, over the mainland border, to assist in arranging a meeting with Lee.

Last week, Chinese state broadcaster CCTV showed a tearful Gui Minhai, the co-owner of Might Currents, who disappeared from Thailand in October, making a televised confession to a hit-and-run case in the city of Ningbo in 2003. There is still no news from the three other missing booksellers, who were last seen in Shenzhen in October, where they went to visit their wives.

Associated Press contributed to this report