"Schools will know for sure, people on the outside like us should not comment on which [song] is allowed or not," he said during a radio programme.

The comments come a day after Yeung sent a letter to schools saying students should be reminded not to boycott classes, shout slogans in schools, form human chains, put up posters or even sing any songs with political messages.

When asked on Commercial Radio to clarify which political songs would not be allowed, Yeung said "well, it depends, as in the future there might be new songs".

But when asked if a song like "Glory to Hong Kong" would be banned, he replied "this song is obviously a political propaganda song".

When asked if songs from Les Miserables would be allowed at a school music competition, he replied: "I think that would be allowed, but of course it would depend on the environment, and the purpose."

"If this action disrupts campus, affects the emotions of students, or encourages students to take part in some activities, then its purpose is political propaganda."

When asked if "Love Basic Law" – a Cantopop song released in 2006 to celebrate the 16th anniversary of the promulgation of the Basic Law – would be allowed, Yeung replied that loving the Basic Law is not a political action, is something that everyone should do like respecting your elders, and therefore not propaganda.

On Wednesday, Yeung sent a letter to primary and secondary school principals across the territory telling them to discipline students who participate in a possible class boycott mulled by a student group.

The group is holding what it calls a referendum among students to decide whether to hold the action. Some trade unions are also conducting a similar exercise to see if they should go ahead with a general strike over the new security law planned by Beijing.

Yeung in his letter had said that students who are found to be involved in any way in this "referendum" – like taking part, or encouraging others to do so – must be told to stop.