This is a very significant judgment that effectively redefines two sections of the Terrorism Act 2000 that have until now been used increasingly by the police and prosecutors .

The judges, led by the Lord Chief Justice, said that the police had been casting their net far too wide in deciding what amounted to terrorist material. The effect of their judgment is that it is no longer illegal for someone to have in their possession al-Qaeda propaganda.

Sections 57 and 58 relate to possessing items of potential use to terrorists. The authorities have been using these clauses to justify arresting people who they think are being radicalised and who might be persuaded to travel overseas to fight jihad or attend terror training camps. The legislation has been seen as a way to nip terrorist activities in the bud.

The Appeal Court has now said that it is not sufficient to show that someone is simply reading or possessing extremist literature. The test in future will be whether the suspect has a clear intent to engage in terrorism.

What this ruling says is that the possession of Islamist propaganda in the form of books, videos and websites is no longer an offence, unless it can be demonstrated unequivocally that there is an intention to commit a terrorist act.

The Crown has asked for seven days to consider an appeal to the House of Lords. It regards this ruling as a serious blow to the architecture of terrorist legislation in the UK — a body of law which the authorities believe has been highly effective in thwarting terrorist attacks and curtailing the radicalisation and recruitment process.

To human rights lawyers, this is a victory for freedom of thought and a reminder to the State that the law has to be quite strictly interpreted: it cannot be used as a means of criminalising people for having radical thoughts or exploring extreme ideologies.

The ruling could have implications for about half a dozen cases currently going through the courts. It will almost certainly lead to an appeal in the case of Samina Malik, the so-called Lyrical Terrorist.

And it will force police to rethink how they proceed in future investigations, particularly where they are trying to clamp down on radicalisation.

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