Boris Johnson has said he is "deeply concerned" by China's approval of a controversial law which would allow authorities to crack down on subversive and secessionist activity.
The prime minister said: "We are obviously deeply concerned about the decision to pass the national security law in Beijing as it affects Hong Kong.
"We will be looking at the law very carefully, and we will want to scrutinise it properly to understand whether it is in conflict with the Joint Declaration between the UK and China.
"And we will be setting out our response in due course."
The South China Morning Post newspaper and public broadcaster RTHK, both citing unnamed sources, said that the Standing Committee of the National People’s Congress voted unanimously to approve a national security law for Hong Kong on Tuesday.
The legislation is aimed at curbing subversive, secessionist and terrorist activities, as well as foreign intervention in the city’s affairs.
It follows months of anti-government protests that at times descended into violence in Hong Kong last year.
Legislative Councillor for Hong Kong, Tanya Chan, warned the new law is a "white terror" that could spread to all areas of life on the island.
ITV News Asia Correspondent Debi Edward on the developments in Hong Kong
"I think this white terror will spread to all aspects of our normal lives and will affect not only the politicians, it will also affect normal families with kids," she told ITV News.
"Because based on the definition of national security in China, under the legislation, you can see this has very wide coverage.
"For example, economies, education, religion and all these will be affected so I’m easily worried about how far it can go.
"It seems that this piece of law is targeting just a small number of people but I don’t think so because, as I’ve mentioned it’s like a white terror - it will just spread on and will affect different types of people."
Hong Kong resident Oscar Or, who has a baby son, said he has been angered by the new law and is considering moving to the UK.
"We believe that in the UK they have better education environments to provide to my son, because these two years we notice that in schools… we feel that it is changing," he told ITV News.
"The books have been edited by the government, the history has changed slightly, so we think it’s time to go somewhere for my son, for my family, and for my future.
"We are a bit sad to make this decision, even though now is not our final decision, we believe this is the last option for us.
"We are still looking for an opportunity to stay in Hong Kong because Hong Kong is our home."
In a statement issued on Tuesday, Dominic Raab said: "Despite the urging of the international community, Beijing has chosen not to step back from imposing this legislation.
"China has ignored its international obligations regarding Hong Kong. This is a grave step, which is deeply troubling.
"We urgently need to see the full legislation, and will use that to determine whether there has been a breach of the Joint Declaration and what further action the UK will take."
Tam Yiu-Chung, Hong Kong’s sole representative to the Standing Committee of the National People’s Congress, confirmed in an interview with reporters Tuesday that the law had been passed.
He said punishments would not include the death penalty, but did not elaborate on further details such as whether the law could be applied retroactively.
“We hope the law will serve as a deterrent to prevent people from stirring up trouble,” Tam said in the interview.
“Don’t let Hong Kong be used as a tool to split the country.”
Hong Kong leader Carrie Lam earlier declined to comment on the law at a weekly meeting with reporters, saying it was inappropriate for her to do so while the Standing Committee was still meeting.
She did say that once the law is passed, “the Hong Kong government will announce it and promulgate it for implementation here, and then I and my senior officials will do our best to respond to everyone’s questions, especially regarding the enforcement of this national law.”
The law has met with strong opposition within Hong Kong and condemnation from former colonial ruler Britain, the US, the European Union and others.
Human rights groups have warned the law could target opposition politicians seen as insufficiently loyal to Beijing for arrest or disqualification from running in September elections for the Legislative Council.
Ahead of the announcement, the US government said on Monday it will bar defence exports to Hong Kong and will soon require licenses for the sale of items to Hong Kong that have both civilian and military uses.
The administration has warned for weeks that if the law was passed, it would take action to end special US trade and commercial preferences Hong Kong had enjoyed since reverting to Chinese rule in 1997.
“The United States is forced to take this action to protect US national security,” Secretary of State Mike Pompeo said in a statement.
“We can no longer distinguish between the export of controlled items to Hong Kong or to mainland China. We cannot risk these items falling into the hands of the People’s Liberation Army, whose primary purpose is to uphold the dictatorship of the (ruling Communist Party) by any means necessary.”
The US Senate on Thursday unanimously approved a bill to impose sanctions on businesses and individuals — including the police — that undermine Hong Kong’s autonomy or restrict freedoms promised to the city’s residents.
Britain says it could offer residency and possible citizenship to around three million of Hong Kong’s 7.5 million people.
China has denounced all such moves as gross interference in its internal affairs and foreign ministry spokesperson Zhao Lijian on Monday said Beijing has decided to retaliate with visa restrictions on “US personnel who perform badly on Hong Kong related issues”.
China decided to use the National People’s Congress to enact the legislation after opposition within Hong Kong’s Legislative Council and within society as a whole made it impossible to pass at the local level.
The law is seen as the most significant erosion to date of Hong Kong’s British-style rule of law and high degree of autonomy that China promised Hong Kong would enjoy at least through 2047 under the “one country, two systems” framework.
Passage of the legislation will also allow the central government in Beijing to set up a national security office in Hong Kong to collect and analyse intelligence and deal with criminal cases related to national security.