The world is watching Hong Kong as a "test case" of whether Chinese promises mean anything, Britain's last governor said ahead of the 20th anniversary of his tearful departure from the city.

Chinese President Xi Jinping will visit Hong Kong to mark the anniversary of its handover from British rule on July 1, and Chris Patten said Xi should reaffirm Beijing's respect for the city's unique brand of autonomy dubbed "one country, two systems".

By and large, he said, China had respected its treaty obligations to Hong Kong and Britain in the years after the 1997 handover.

But more recently, under Xi, Patten said: "I think there's been more and more indication of Beijing tightening its grip on Hong Kong and on Hong Kong's windpipe."

Building up to Xi's visit, Hong Kong has seen multiple protests by pro-democracy activists as concerns rise that Beijing is trampling over the handover agreement by interfering in a range of areas, from politics to the judiciary, education and media.

Xi's visit will be his first since becoming president in 2013 and will culminate with the inauguration of Hong Kong's new leader, Carrie Lam, on Saturday.

"It'll be interesting to see how well that's received or how well he's protected from a dialogue with people in Hong Kong," Patten told reporters in London.

He said it was "preposterous" of Lam to claim last week that the case of five booksellers who were allegedly abducted by Chinese police two years ago had nothing to do with Hong Kong's government.

- 'Routine humiliations' -

London protested over the case of one of the booksellers, who holds a British passport, but Patten said such complaints were merely an "admonitory clearing of the throat".

"It's all part of this illusion that the only way you can do business with China... is by bowing however low the Chinese say they want you to do," he said.

Patten is promoting a new memoir called "First Confession", which deals at length on his years of doing battle with Beijing when he promoted a measure of democratic reform in Hong Kong prior to the handover.

Those years were an exception to what he writes were Britain's "routine humiliations" at Beijing's hands in the 1980s and early 1990s -- and Patten frets that Britain's exit from the EU and the ensuing need to develop more trade outside Europe could make the country even less likely to tussle with China in future.

But Patten, who looks back at his governorship as the best years of his life, said trade was an example of where China does not always respect its promises.

European companies struggling to do business in China had already learned the hard way that Beijing's commitments to reform were not iron-clad, the former EU commissioner said at the press briefing.

Failure to abide by the promise to respect Hong Kong's basic freedoms would be a much bigger violation and would give the world pause for thought, he said.

"The way it meets its obligations contained in a treaty lodged at the United Nations ... the way it meets its obligations to Hong Kong, will be taken by many people as a test case of whether China's word is something that people can take as gospel." afp