Foreign Secretary Dominic Raab has accused China of a “clear and serious violation” of a treaty forged with the UK by imposing national security legislation on Hong Kong.

The Cabinet member said the UK will “honour” its commitment to citizens of the former British colony, which could extend the right for nearly three million Hong Kongers to come to the UK.

Mr Raab on Wednesday said Beijing had breached the Joint Declaration signed between the UK and China to protect freedoms in the territory by enacting the controversial legislation.

His statement came after Hong Kong police made their first arrests under the law, including one person said to have displayed a sign with the Union Jack and calling for Hong Kong’s independence.

Taking effect on Tuesday night, the law makes activities deemed subversive or secessionist punishable by imprisonment. It is seen as targeting the anti-government demonstrations.

In a statement outside the Foreign Office, Mr Raab told reporters the UK had carefully assessed the legislation since its publication on Tuesday night.

“It constitutes a clear violation of the autonomy of Hong Kong and a direct threat to the freedoms of its people, and therefore I’m afraid to say it is a clear and serious violation of the Joint Declaration – the treaty between the United Kingdom and China,” he said.

Mr Raab, who was due to make a statement to MPs in the House of Commons later in the day, said he will “honour” the UK commitment to those with British National (Overseas) nationality, or BNO.

He accused China of not “living up to its promises” over the freedom for people to peacefully protest in Hong Kong, adding: “We will live up to our promises to them.”

More than 70 arrests were made in Hong Kong on Wednesday, which marked 23 years since the handover.

Prime Minister Boris Johnson had pledged to upgrade the status of BNO passports held by 350,000 people in Hong Kong, with a further 2.5 million eligible to apply.

He said that if China enacted the law, the nationals would be granted immigration rights beyond the current six-month limit.

Mr Johnson is under pressure from across the political spectrum to take a firmer stance against Beijing, including over the role of Chinese firm Huawei in the 5G network.

And he is facing calls to act over the breach of the 1985 Sino-British Joint Declaration, the legally-binding agreement to give Hong Kong a level of autonomy for at least 50 years.

Ahead of Mr Raab’s statement, his Labour shadow, Lisa Nandy, called on the Government to “lay out the concrete steps” to fulfil its commitments to the people of Hong Kong.

“Now is not the moment to look away,” she said.