A man who is serving a prison term in Hong Kong has been blocked from having his sentence reduced under a new security legislation that went into effect on Saturday.

Prisoners in Hong Kong can have their jail terms shortened for good behavior.

But under the new legislation, sentences are reduced only if the prison administrator judges that an inmate jailed for endangering national security is not a risk.

Hong Kong media report that the revised rule was applied for the first time on Monday to a man who was convicted of inciting secession by calling for Hong Kong's independence in 2020.

Hong Kong's Chief Executive John Lee told reporters on Tuesday that endangering national security is a serious crime, and that perpetrators should not expect to be granted an early release from prison.

He warned people not to try any acts or activities that put national security at risk.