Boris Johnson has promised to change Britain's immigration rules for the people of Hong Kong as Beijing continues to tighten its grip on the former British colony. 

In a letter to the people of Hong Kong published in the South China Morning Post, the Prime Minister said the UK was prepared to "provide an alternative" for citizens fearing for their way of life under increasing Beijing control. 

Mr Johnson warned that if China goes ahead with the controversial national security law - a piece of legislation that will make it illegal to undermine Beijing - Britain will change the rules for holders of the British National Overseas (BNO) passport. 

Under current rules, BNO passport holders are allowed to come to the UK visa-free for up to six months but they do not have the right to work. 

The Prime Minister said the rules would be changed to allow the some 350,000 BNO passport holders in Hong Kong to come to the UK for up to 12 months with the right to work.  

This would then "place them on a route to citizenship", Mr Johnson said. 

While these passports were issued to Hong Kongers before the 1997 handover, the Prime Minister said the door would be opened for another 2.5 million to apply for them. 

"This would amount to one of the biggest changes in our visa system in British history. If it proves necessary, the British government will take this step and take it willingly," he said. 

"Many people in Hong Kong fear their way of life – which China pledged to uphold – is under threat. If China proceeds to justify their fears, then Britain could not in good conscience shrug our shoulders and walk away; instead we will honour our obligations and provide an alternative.

"I hope it will not come to this. I still hope that China will remember that responsibilities go hand in glove with strength and leadership." 

The Prime Minister's statement has been welcomed by democracy campaigners. 

Benedict Rogers, East Asia Team Leader at Christian Solidarity Worldwide, thanked the Prime Minister for standing with Hong Kong.

"Today Britain truly stood up for Hong Kong," he tweeted. 

Johnny Patterson, Director of Hong Kong Watch said: "This is a remarkable intervention from the Prime Minister. 

"It is a watershed moment in Sino-British relations - no sitting PM has made a statement as bold as this on Hong Kong since the handover.

"It reflects two things, first the severity of the situation on the ground, and second the fact that the British government genuinely, and rightly, feel a sense of duty to citizens of Hong Kong and are going to do all they can to stop them becoming the collateral damage of escalating geopolitical tensions."

The statement from the Prime Minister comes after hundreds of international leaders and dignitaries, including Church of England bishops and the former Archbishop of Canterbury, Dr Rowan Williams, signed a global statement condemning the national security law. 

"We, the co-signed, write to express grave concerns about the unilateral introduction of national security legislation by Beijing in Hong Kong," the letter reads.

"This is a comprehensive assault on the city's autonomy, rule of law, and fundamental freedoms. The integrity of one-country, two-systems hangs by a thread.

"It is the genuine grievances of ordinary Hong Kongers that are driving protests. Draconian laws will only escalate the situation further, jeopardising Hong Kong's future as an open Chinese international city.

"If the international community cannot trust Beijing to keep its word when it comes to Hong Kong, people will be reluctant to take its word on other matters.

"Sympathetic governments must unite to say that this flagrant breach of the Sino-British Joint Declaration cannot be tolerated."

The Catholic Church in England and Wales' chief spokesman on international affairs, Bishop Declan Lang, this week called on the UK Government to fulfil its responsibilities under the Sino-British Joint Declaration.

"Like so many others in the Catholic community, I am deeply concerned by the continuing erosion of autonomy, suppression of political freedoms, and violent response to peaceful protests taking place in breach of this treaty," he said.

"The UK has a clear legal, moral and historical duty to safeguard fundamental freedoms in Hong Kong.

"Failure to do so at this critical time will not only have devastating consequences for more than seven million people living there but is also likely to have dangerous repercussions for human rights and international law more broadly."