PM Boris Johnson said earlier on Wednesday that if China pursues hotly-debated national security laws in Hong Kong, millions of its residents will be eligible for the British National Overseas (BNO) passport.

The passport currently allows holders to enter the UK without a visa for six months, but British immigration laws could be changed in such a way that overseas nationals would be able to stay in the UK for a renewable period of 12 months – and become British citizens in the future.

On Thursday, Chinese diplomats hit back at the PM’s promise, saying it was an affront to its sovereignty and a breach of London’s own agreements with Beijing. The UK had once declared that “it will not confer the right of abode to Chinese citizens in Hong Kong who hold BNO passports,” China’s Embassy in London recalled.

If Hong Kongers are allowed to emigrate en masse, it would breach the UK’s “own position and pledges as well as international law and basic norms governing international relations,” it added.

We firmly oppose this and reserve the right to take corresponding measures.

Ambassador Liu Xiaoming noted that nothing in the joint Sino-British declaration returning Hong Kong to China gives the UK “sovereignty, jurisdiction or right of supervision” over the former colony.

Echoing countless statements by Chinese officials, he said Hong Kong affairs fall within China’s jurisdiction and “brook no external interference.”

“Gone are the days when Hong Kong was under British colonial rule,” he tweeted.