HONG KONG (Reuters) - “Important areas” of Hong Kong’s “One Country, Two Systems” framework are coming under growing pressure, illustrated by developments such as reports of mainland security officials operating in the autonomous city, a British government report said on Thursday.

FILE PHOTO - A ferry sails at Victoria Harbour in front of the financial Central district, featuring AIA Central (C) and Cheung Kong Center behind it, in Hong Kong, China February 17, 2016. REUTERS/Bobby Yip/File Photo

The “One Country, Two Systems” principle, implemented as the former British colony returned to Chinese rule in 1997, promises the city a high degree of autonomy, an independent judiciary and a range of freedoms not enjoyed in mainland China.

The former colonial power has been issuing reports on Hong Kong every half a year since then, and the latest, which covers developments in the first six months this year, British Foreign Secretary Boris Johnson said the guiding principle has “generally functioned well.”

“However, at the same time, we cannot ignore that important areas of the ‘One Country, Two Systems’ framework are coming under increasing pressure,” Johnson wrote.

Examples include “further reports of mainland security officials operating within” Hong Kong, he said, without elaborating, as well as reports of the local Beijing representative’s office heightening its influence in the city.

But for the first time, the half-yearly report stopped short of raising concern over five Hong Kong booksellers who disappeared in late 2015 and later mysteriously re-emerged in mainland Chinese custody. One of the five men, Swedish citizen Gui Minhai, is still in Chinese detention.

The report also touched upon the “apparent abduction” of Chinese billionaire Xiao Jianhua, who in January disappeared from his downtown Hong Kong apartment in a wheelchair with his head covered.

Although Hong Kong authorities said there was no evidence to suggest mainland law enforcement agents had acted on Hong Kong soil, Johnson noted that “many in Hong Kong and internationally highlighted the numerous similarities between Xiao Jianhua’s apparent abduction and the case of the Hong Kong booksellers.”

He also said he looked forward to welcoming the city’s newly sworn-in leader, Carrie Lam, to visit London to discuss closer co-operation between the UK and Hong Kong “as the UK prepares to leave the EU.”

In response to the report, the Hong Kong government said foreign governments should not interfere in the internal affairs of the city.

“Since the return to the motherland, the HKSAR (Special Administrative Region) has been exercising a high degree of autonomy ... This demonstrates the full and successful implementation of the ‘one country, two systems’ principle, which has been widely recognized by the international community.”