The UK attorney general has warned Twitter and Facebook users about the dangers of committing contempt of court when posting comments online.

Advisory notes, which are already issued confidentially to print and broadcast media about controversial or emotive trials, will now be on the Internet and Twitter to help prevent users from publishing information that could threaten a fair trial.

Dominic Grieve QC said: "Blogs and social media sites like Twitter and Facebook mean individuals can now reach thousands of people with a single tweet or post.

"In days gone by, it was only the mainstream media that had the opportunity to bring information relating to a court case to such a large group of people that it could put a court case at risk. That is no longer the case and is why I have decided to publish the advisories that I have previously only issued to the media."

The growth of social media sites and users publicising their thoughts more than ever before could lead to challenges for the criminal justice system.

In the past, judges have also had to abandon trials because of misleading articles being published, though no Twitter storm has yet stopped a court hearing.

Last year nine people were prosecuted for posting the name of the woman raped by former Sheffield United footballer Ched Evans, on various social media sites.

Grieve has also taken action against three men who used Twitter and Facebook to publish photographs claiming to be of Jon Venables and Robert Thompson, the murderers of the toddler James Bulger, as grown- ups.

The publicised distribution of legal warnings could make it harder in future for users of Facebook or Twitter to claim they made comments in ignorance of the law.

Are there problems concerning internationalism considering Twitter’s global reach? Will the Will the Attorney General be able to control what’s tweeted from countries outside the UK?