The national security law that China has imposed on Hong Kong is already curtailing speech in the territory - but it is having a far wider impact.

It applies to everyone in the world, everywhere in the world. People who break the law can be prosecuted if they go to Hong Kong.

That has brought an unexpected headache for foreign universities, which are scrambling to work out how to protect their students from saying and writing things that might later be used against them.

Foreign institutions renowned as bastions of free speech are having to deal with Chinese censorship.

Anyone who criticises China and travels to Hong Kong is potentially at risk of arrest under the new law.