The departure of Hale, a former president of Britain's Supreme Court, comes amid international concern over the effect on the city of a national security law imposed by China's parliament a year ago.

Diplomats and businesspeople say the law's impact on Hong Kong's independent judges and separate legal system is being particularly closely watched given their importance to its status as a global financial hub.

Hale is one of 13 overseas non-permanent judges on the top court - a presence that has long been seen as a symbol of the rule of law after Britain handed the former colony back to China in 1997.

The Hong Kong Judiciary said in a statement that Hale "has

indicated to the Judiciary that for personal reasons she would not wish to have her appointment" extended for another term.

Overseas judges' "immense contribution to Hong Kong has repeatedly been acknowledged", the Judiciary said.

Hale, 76, was quoted in London's The Times newspaper on Friday as telling an online forum that the security law - imposed by China's parliament last June - had left "all sorts of question marks up in the air".

"The jury is still out on how they will be able to operate the national security law," she was quoted as saying. She said her main reason for quitting was COVID-19 travel restrictions that meant she could not travel easily to Hong Kong.

Hale could not immediately be reached for comment.

The security law gives Hong Kong Chief Executive Carrie Lam the power to select judges for a roster of jurists that will handle national security cases.

In the most serious cases suspects can also be taken to mainland China for trial while law also grants extensive powers to personnel from Beijing's security apparatus, who are now based in the city for the first time.

The current president of Britain's Supreme Court, Robert Reed, is also on the Court of Final Appeal and has met British Foreign Secretary Dominic Raab to discuss the situation under the new law.

Reed told a House of Lords' panel in March he would not be prepared to serve or nominate any of his judges to serve if there was "any undermining of the independence of the Hong Kong judiciary".

Local lawyers monitoring the situation said on Friday that Hale would ordinarily be expected to continue, so her looming departure was a blow to the system.

Australian judge James Spigelman resigned last September, citing the law in a comment to Australia's national broadcaster. Other foreign judges have since extended their terms or joined the court.

Lam appointed Hale in 2018 together with former Canadian Chief Justice Beverly McLachlin as the first women to serve on the top court.