Beijing imposed the national security law on Hong Kong in 2020 after months of huge and often violent pro-democracy protests in the former British colony.
In 2021, Chung, then 20, became the youngest person to be jailed under the security law after he pleaded guilty to “secession” and was sentenced to 3½ years in prison.
Chung said he had “safely arrived in the United Kingdom and have formally applied for political asylum upon entry”, according to a statement posted on Facebook early Friday but dated December 27.
He said he had been closely watched by Hong Kong’s national security police despite his jail term having ended in June and was blocked from taking up work.
The Washington Post reported he was released early for good behavior.
Authorities also stopped him from seeking legal assistance by citing a confidentiality clause under the security law, Chung added.
He said he got permission to leave Hong Kong by saying he wanted to go on holiday in Okinawa, Japan, and sought help once he was outside Chinese soil.
Calling himself a “Hong Kong exile,” Chung said it was impossible for him to return home in the foreseeable future.
“I believe that only with the collective efforts of the Hong Kong people can Hong Kong once again become a home that we can be proud of,” he wrote.
“I believe that as long as the Hong Kong people never give up, the seeds of freedom and democracy will sprout alive again.”
AFP has contacted the police for comment.
Chung was previously the convenor of Student Localism, a small group he set up five years ago as a secondary school pupil to advocate for Hong Kong’s independence from China.
Separation from China was then a fringe view in Hong Kong although calls for self-rule became more vocal during the 2019 protests.
In 2020, Chung was nabbed by plainclothes police from a coffee shop opposite the U.S. Consulate, where he was allegedly planning to seek asylum.