The former governor of Hong Kong, Chris Patten, says the UK risks not meeting its promises to the territory and "selling its honour" in an attempt to reach trade deals with China.
Speaking to BBC Newsnight, Lord Patten said the UK had let down "a generation" of democracy activists.
It is 20 years since Hong Kong was returned to China after more than a century of British rule.
The UK government says it takes its commitments to Hong Kong seriously.
Anson Chan, former Hong Kong chief secretary - who worked as Lord Patten's deputy - also expressed deep concern about China's behaviour towards Hong Kong.
Citing the example of the alleged kidnapping by China of five booksellers and other rights abuses, she told BBC Newsnight that the "one country, two systems" form of rule itself is under threat.
"Unfortunately the rest of the world - particularly Great Britain - would rather pretend not to see what is going on," she said.
"If they continue to ignore this steady erosion, by the time they wake up to the fact that 'one country, two systems' exists only in name, it will be too late."
In the 1980s the Chinese and British leadership agreed that Hong Kong would be guaranteed certain freedoms not enjoyed in the rest of China - freedom of press, freedom of assembly and a partially-elected law-making council.
This principle, known as "one China, two systems", was a part of the Sino-British joint declaration - an international agreement guaranteeing Hong Kong those freedoms after the handover.
Lord Patten said the UK government has not "manifestly stood up for Hong Kong".
"I wonder what has happened to our sense of honour and our sense of responsibility - particularly in Britain. It's above all a British question," he said.
"We signed the joint declaration with China. It's a treaty at the UN. It's supposed to commit us to standing up for Hong Kong's rights until 2047."
"And you don't get much sense of the British government actually standing over those promises and obligations and I think that's a great pity."
'Kowtowing' to China
Lord Patten said the UK risks putting its desire to do trade with China, over its commitment to Hong Kong.
"It's all for derisory, ludicrous reasons," he said. "The argument that the only way you can do trade with China is by kowtowing to China on political issues is drivel - it's complete nonsense."
"I worry about how people are prepared to sell our honour for alleged trade deals which never actually happen. I think that that would be calamitous. And what do we represent to the world if that's what happens?"
In 2015, five publishers selling critical articles about the Chinese leadership disappeared, only to reappear in detention in the mainland.
One bookseller was thought to have been abducted while in Hong Kong. Four of the publishers - including a British passport-holder - were eventually returned to Hong Kong. One Swedish national remains in Chinese detention.
Tens of thousands of protesters took to the streets of Hong Kong in 2014 in what came to be known as the "Umbrella Protests". The protests lasted several weeks, and captured the world's attention, but failed to achieve any concessions from Beijing.
" I feel very strongly that we let down the parents of this generation of democracy activists. I think it would be a tragedy if we let down these kids as well," Lord Patten said.
A Foreign Office spokesperson said: "The UK takes our longstanding commitment under the Sino-British Joint Declaration very seriously."
"We believe that 'one country, two systems' continues to be the best arrangement for Hong Kong's long term stability and prosperity, as it has been for nearly 20 years.
"We hope and expect that 'One Country Two Systems' will be respected and successful long into the future."
The spokesperson added: "We regularly discuss the importance of respect for 'one Country, two Systems' and Hong Kong's high degree of autonomy with the Chinese Government. The Foreign Secretary made this clear to his Chinese counterpart when they met in London in December."
More on this story on BBC Newsnight at 22:30 on BBC Two