The persecution of Christians in mainland China is spreading to Hong Kong, Release International has warned on the 35th anniversary of the Tiananmen Square massacre.

The massacre in Beijing's Tiananmen Square on 4 June 1989 brought a brutal end to pro-democracy protests and marked an increase in the persecution of Christians.

Release International said that 35 years on, Christians in China are facing the worst levels of persecution since the Cultural Revolution and that the threat is spreading to Hong Kong, where national security laws have had a chilling effect on free speech and religious freedom.

The organisation, which supports persecuted Christians worldwide, said that a new law could force Catholic priests in Hong Kong to reveal the secrets of the confessional.

Under Article 23, passed in March, priests could be jailed for up to 14 months if they refuse to disclose so-called crimes of treason shared during confession. 

Release International partner Bob Fu said that if priests were forced to violate the trust of Catholics coming to confession, "China will go down a very dangerous path towards persecution."

Fu, who has spent years campaigning for religious freedom in China, said that many Christians had already left Hong Kong and that "their preferred destination is the United Kingdom".

He said that Britain has a moral obligation to stand up for religious freedom in its former colony. 

"Hong Kongers are expecting the UK to stand strong for their religious freedom and to speak up for them, and to take all necessary measures to protect those who flee persecution," he said. 

A new report by the United States Commission on International Religious Freedom (USCIRF) says that China is tightening its grip on practising Christians, and that believers in Hong Kong and beyond China's national borders are being affected. 

Release International CEO Paul Robinson said, "The long-running crackdown on mainland China now appears to be extending to Hong Kong.

"Religious freedom is the cornerstone of all freedoms. Our partners describe the current crackdown on Christians as the harshest since Mao Tse Tung's Cultural Revolution.

"Together we call on the world to wake up and recognise the severity of the persecution in China that is gathering pace. This threat against Christians goes beyond their national borders."