By John Sudworth for BBC News
A former employee of the UK's Hong Kong consulate has told the BBC that he was tortured in China and accused of inciting political unrest in the city.
Simon Cheng, a Hong Kong citizen who worked for the UK government for almost two years, was detained for 15 days on a trip to mainland China in August.
"I was shackled, blindfolded and hooded," the 29-year-old told the BBC.
UK government sources say they believe his claims - of being beaten and forced to sign confessions - are credible.
Following the BBC interview, the British Foreign Secretary Dominic Raab summoned the Chinese ambassador.
"We are outraged by the disgraceful mistreatment that Mr Cheng faced when he was in detention in mainland China ... and we've made clear that we expect the Chinese authorities to review and hold to account those responsible," Mr Raab said.
On Wednesday a Chinese foreign ministry spokesperson told the BBC they "absolutely cannot accept the UK government's interference in this case" - and would in return summon the UK ambassador to "express their opposition and anger".
"We hope the UK will be prudent and stop interfering in Hong Kong and in China's domestic affairs because it will, eventually, only harm the UK's own interests," the spokesperson added.
Mr Cheng's raises questions for both the Chinese and the UK governments.
The claims he makes - including that he saw other Hongkongers in Chinese custody - are likely to fuel protesters' fears that their city's freedoms are being eroded under Chinese rule.
"They said they work for the secret service and that there are no human rights," he tells me. "Then they started the torture."
Who is Simon Cheng?
As a trade and investment officer at the UK consulate, Simon Cheng's particular brief was to drum up interest in investing in Scotland among the Chinese business community.
It required him to travel frequently to mainland China.
In June, with Hong Kong engulfed in mass demonstrations, Mr Cheng volunteered for an additional role.
"The British Consulate instructed staff to collect information about the status of the protests," he says.
As a supporter of the pro-democracy movement he found it easy to blend in and, with the consent of the consulate, he signed up to some of the social media groups through which the protesters co-ordinated their actions.
Paid overtime for the information he gathered, he began reporting back what he saw to his colleagues.
His task, both Mr Cheng and UK government sources insist, was not to direct events in any way but to purely observe - the kind of civil society monitoring work many embassies do.
China had already begun to accuse the UK of meddling in Hong Kong, with British politicians becoming increasing vocal in their support for the protesters.
On 8 August, with emails still on his phone linking him to that work observing the protests, he was sent by the consulate to a business conference in the Chinese city of Shenzhen.
How did he disappear?
Although China has ruled Hong Kong for more than 20 years, the border between the city and the mainland still looks and feels like an international boundary.
The "one country two systems" principle - that the protesters say they are fighting to preserve - is meant to ensure that Hong Kong retains control over most of its affairs, including its borders.
But Simon Cheng was about to discover for himself the blurred edges of that legal and political framework.
Since the opening of the Hong Kong-Shenzhen high-speed rail link last year, a new border post has been placed inside West Kowloon station, in the heart of Hong Kong.
It is deeply controversial: Hong Kong's pro-democracy movement views the presence of the Chinese police, manning the Chinese side, as an unwelcome extension of Chinese authority.
It was here they stopped Simon Cheng, returning from his business trip.
He was put on a train, transported back to Shenzhen and handed over, he says, to three plainclothes officers from China's National Security Police.
How does he describe his ordeal?
Raising his arms above his head, Mr Cheng showed how he was hung up from a chain linking the handcuffs on his wrists.
The questions focused on his involvement in the protests with the aim, he says, of forcing him to confess to fomenting unrest on behalf of the British state.
"They wanted to know what role the UK had in the Hong Kong protests - they asked what support, money and equipment we were giving to the protesters."
He said he was made to hold stress positions - squatting against a wall for example - for hours on end, and beaten if he moved.
"They would beat the bony parts, like my ankles... or any vulnerable part."
He claimed he was subjected to sleep deprivation, with his interrogators forcing him to sing the Chinese national anthem to keep himself awake.
He believes he was not the only Hongkonger undergoing such treatment.
"I saw a bunch of Hong Kong people getting arrested and interrogated. I heard someone speak in Cantonese saying: 'Raise your hands up - you raised the flags in the protest didn't you?'"
Watch: Video journalist Dom Thomas explains the background to the Hong Kong protests