The British government on Thursday urged Hong Kong to halt prosecutions under its National Security Law and release 14 pro-democracy campaigners found guilty of subversion.

"We call on the Hong Kong authorities to end NSL prosecutions and release all individuals charged under it," junior foreign minister Anne-Marie Trevelyan said in a statement.

Britain handed back Hong Kong to China in 1997 but has become increasingly critical of Beijing's influence on its former colony, accusing it of breaking its promise to protect democratic freedoms.

Relations have soured between the two capitals, including after Hong Kongers were given residency and a route to citizenship in the UK due to the crackdown on pro-democracy campaigners.

Trevelyan said Thursday's verdict was "a clear demonstration of the way that the Hong Kong authorities have used the Beijing-imposed National Security Law to stifle opposition and criminalise political dissent".

The 14 people found guilty, who were among 47 charged, were "guilty of nothing more than seeking to exercise their right to freedom of speech, of assembly and of political participation", she said.

"Today's verdict will only further tarnish Hong Kong's international reputation. It sends a message that Hong Kongers can no longer safely and meaningfully participate in peaceful political debate."