The outgoing British prime minister, Theresa May, has told Beijing that the Sino-British Joint Declaration, which paved the way for Hong Kong's 1997 handover, is still in force and must be respected.

Speaking at her last major public engagement, at the Chatham House think tank in London, May hailed Britain's growing trade links with China.

A diplomat from China's embassy in London questioned the prime minister about her advice to her successor on maintaining a relationship with Beijing, with particular reference to disputes over Hong Kong.

May said there were areas on which Britain needed to be "very clear" with Beijing and "the continuation of the Joint Declaration in relation to Hong Kong is one of those".

"That declaration continues to be in force," she said. "We would say to China that it needs to be abided by, it needs to be respected, and continue to be respected.

"So, in a sense it's like all relationships. We look to develop those economic ties, but we will also be very clear when we feel that there are messages we need to give, and I'm sure that will continue into the future."

Some Beijing officials have been dismissive of the Joint Declaration. Foreign Ministry spokesman Lu Kang described it in 2017 as a "historical document" that no longer had any realistic meaning.

May's governing Conservative Party is in the process of electing a replacement for her as leader. Foreign Secretary Jeremy Hunt, who has been highly critical of the Hong Kong government's attempts to change the extradition law, is one of two candidates along with the favourite, his predecessor Boris Johnson, a former London mayor.-RTHK