BRUSSELS/TOKYO (Reuters) - The European Union expressed anger while Britain and Japan voiced concern on Tuesday after China passed a national security law for Hong Kong that threatens to usher in the starkest changes for the former British colony since its return to China.

The law approved earlier by China’s mostly rubber-stamp parliament, follows months of pro-democracy protests in Hong Kong, which was guaranteed freedoms not enjoyed on the mainland under a “one country, two systems” formula at its 1997 handover.

“We deplore the decision,” EU Council President Charles Michel told a news conference.

“This law risks seriously undermining the high degree of autonomy of Hong Kong and having a detrimental effect on the independence of the judiciary and the rule of law.”

Authorities in Beijing and Hong Kong have repeatedly said the legislation is aimed at a few “troublemakers” in Hong Kong and will not affect its rights and freedoms, nor investor interests.

Despite such the assurances, the European Union has warned of serious consequences over the legislation, which democracy activists, diplomats and some businesses say will jeopardise Hong Kong’s semi-autonomous status and its role as a global financial hub.

“We will pay careful attention on how to respond,” said Ursula von der Leyen, the head of the European Commission, adding that the bloc was discussing possible response measures with “international partners” but without giving details.

Last week, the European Parliament urged the bloc to take China to the International Court of Justice in the Hague, the United Nations’ highest legal body, if it went ahead.

In London, British Foreign Secretary Dominic Raab called the legislation a “grave step”.

“We are deeply concerned by unconfirmed reports that Beijing has passed the national security law,” Raab said.

In Tokyo, top government officials called the legislation “regrettable”, saying it undermined credibility in the “one country, two systems” formula.

“We will continue to work with the countries involved to deal with this issue appropriately,” Chief Cabinet Secretary Yoshihide Suga told a news conference.

Japan will also communicate closely with the United States and China, as stable ties between the two global powers were vital for regional and global security, he added.

‘DEEP CONCERN’ Foreign Minister Toshimitsu Motegi told reporters he shared the “deep concern” of the international community and the people of Hong Kong over the measure.

Taiwan’s cabinet said in a statement the new law would “severely impact” freedom, democracy and human rights and Taiwan would continue to offer help to people in Hong Kong.

“The government strongly condemns it and reiterates its support for the people of Hong Kong as they strive for democracy and freedom,” said cabinet spokesman Evian Ting.

“The move severely impacts Hong Kong society’s freedom, human rights and stability.”

Last year’s protests drew wide sympathy in democratic and Chinese-claimed Taiwan, which has welcomed people from Hong Kong who have moved to the island and expects more.

Taiwan President Tsai Ing-wen said she was “very disappointed” by China’s move, adding that it showed the “one country, two systems” formula, which Beijing has mooted as a basis for unification with the mainland, “was not feasible”.

“We hope Hong Kong people continue to adhere to the freedom, democracy and human rights that they cherish,” Tsai told reporters.

Defending the law, Hong Kong’s Beijing-backed leader, Carrie Lam, urged the international community to respect China’s right to safeguard security.

In a video message to the U.N. Human Rights Council in Geneva, Lam said the city of 7.5 million had been “traumatized by escalating violence fanned by external forces”.

“No central government could turn a blind eye to such threats to sovereignty and national security, as well as risks of subversion of state power,” she said.