A says he was tortured after being jailed in China during a business trip.
Foreign Secretary Dominic Raab has demanded a meeting with the Chinese ambassador after Simon Cheng Man-kit claimed he was shackled, blindfolded, hooded and beaten during his 15 days behind bars.
The 29-year-old, who works at the British Embassy in Hong Kong, vanished after crossing into mainland in August.
Officials claimed at the time Mr Cheng was detained for "visiting a prostitute" and had confessed to an "illegal act".
However, during an interview with the BBC he claimed his captors accused him of inciting political unrest in Hong Kong and forced him to sign confessions after being beaten.
He said his captors "said they work for the secret service and that there are no human rights.
"They would beat the bony parts, like my ankles... or any vulnerable part."
Mr Cheng said the Chinese authorities accused Britain of supporting demonstrations in Hong Kong.
He recalled: "They said I'm a state enemy and I'm a traitor, and also they asked whether the consulate instructed me to mingle with the protest."
He added: "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.
"I told them I want to make it 100 per cent clear, the UK didn't assign resources or help with the protests."
Unrest in the city state began in the summer with calls to scrap a now-suspended extradition bill, but have now widened to protest against the way Hong Kong is administered by Beijing.
However, after Mr Cheng's release, his family said he suffered 'no external injuries'.
A statement from the family read: "Thank you for all your concern for Simon, he has returned to Hong Kong safely, and is in the company of consulate staff and friends.
"He has no external injury."
Dominic Raab has said the Government was "shocked and appalled" by Mr Cheng's treatment which he believes "amounts to torture".
He said: "Simon Cheng was a valued member of our team.
"I summoned the Chinese Ambassador to express our outrage at the brutal and disgraceful treatment of Simon, in violation of China's international obligations.
"I have made clear we expect the Chinese authorities to investigate and hold those responsible to account."
Public security authorities in Shenzhen said Simon was released as scheduled after 15 days of administrative detention.
The Scottish Government trade and investment officer was hired locally and did not have a diplomatic passport.
