Legislators have approved a change to the law giving prosecutors more opportunities to appeal against any future acquittals in national security trials.

The Department of Justice will be able to go to the Court of Appeal to challenge acquittals made in cases heard in the High Court by a panel of judges.

By a show of hands on Wednesday, lawmakers also supported changes to the Criminal Procedure Ordinance that will allow the prosecution to lodge a challenge if the judge in a High Court criminal trial with a jury rules that the defendant has no case to answer.

DAB legislator Rock Chen stressed that no one is immune to making mistakes, not even judges, and the risks are too great when it comes to matters of security.

"If the prosecution lacks an avenue of appeal, it means defendants who endanger national security will be escaping the long arm of justice, and it means that we are not able to safeguard national security," Chen said.

Stanley Ng from the Federation of Trade Unions said it is necessary to have extra safeguards when it comes to national security trials.

"We are not saying that the judges deliberately acquit the defendants in such cases. It may be due to the lack of understanding of judges, or they may have a weak awareness of the importance of certain cases," Ng said.

Secretary for Justice Paul Lam told Legco that prosecutors will not abuse their new appeal powers.

"The amendments will not affect the existing rights of the defendant, nor undermine the opportunity to a fair trial and presumption of innocence," he said.