Questions have been raised about Britain’s treatment of a former UK consular worker from Hong Kong, who said he was asked to resign after being detained and allegedly tortured on a work trip to mainland China.

Simon Cheng has been offered a two-year UK visa, but sources said it is a “working holiday” type, which only allows him to spend 12 months employed and leaves him without a path to permanent residency.

It raises the prospect that even if he moves to Britain, he could eventually be forced to return to Hong Kong, where he said he no longer feels safe, in 2021. Critics say Britain should do more to protect ex-diplomatic staff.

“The details of what Mr Cheng says happened to him are heartbreaking and extraordinary: a kidnapping and a forced confession obtained by brutal torture,” said Lord Alton, vice chair of the cross-party Westminster Friends of Hong Kong group.

“The UK government must … begin immediately preparing targeted sanctions, while offering asylum to those seeking to escape the iron grip of dictatorship like Simon Cheng.”

Cheng was seized at the border in late August, on his way back from a work trip to the Chinese border city of Shenzhen. He had been tasked with monitoring the protests in Hong Kong for the British embassy, in addition to his main job promoting trade.

He went public this week with an account of the torture that followed. Over 15 days of detention, it included stress positions, beating and psychological abuse, when he was accused of being a British agent.

He was so frightened after his ordeal that he initially refused to even let the UK consulate issue a statement condemning his treatment, he told the BBC, and has since fled Hong Kong.

Despite initially being granted compassionate leave, he said the UK had come to see him as a security risk because of his long interrogation, and this month he was asked to leave his post. “I was asked to resign on November 2019, which ended my roughly two-year service and employment.”

The British government claims that Cheng left his position voluntarily, and foreign secretary Dominic Raab said supporting the 29-year-old was his “over-riding concern”. He described Cheng’s ordeal as “disgraceful” and has summoned the Chinese ambassador to demand Beijing hold those responsible to account.

But the UK has offered Cheng only a two-year visa, and foreigners need to spend five years in the UK to be eligible for permanent residency.

After Cheng was released in September, he said he felt unsafe in Hong Kong because of threats from Chinese security officials who had originally detained him.

The Chinese authorities warned they could “abduct me back to mainland China in Hong Kong anytime if I don’t behave myself, such as exposing their hidden political motivation and agenda behind my detention to anyone”, he wrote.

Cheng said that he also did a full debrief with senior UK consulate officials after he was freed, and was warned to look out for suspicious people following him. He has now fled to a third country, while other activists are rallying international support.

“That someone could be tortured by a dictatorship and then effectively fired by the UK government is horrific and twisted,” said pro-democracy group Stand with Hong Kong. Asking someone to resign is hardly different to sacking them, not to mention treating Simon as a security risk after his ordeal, they said.

It also called on the government to prepare targeted sanctions like ones passed by the US.