Leading article: Mr Straw has taken a blunderbuss to the fight against terrorism

JACK STRAW'S new Terrorism Bill is not all bad. Terrorism has indeed changed, and there is nothing wrong with treating as terrorists animal-liberation extremists who choose to kill or maim in pursuit of their cause or with granting wider powers to bring to justice murderous anti-abortion activists or groups equivalent to Aum Shinrikyo, the cult that launched deadly poison gas into Tokyo's underground system.

The Metropolitan police were also understandably frustrated by being unable to use the powers - available under the Prevention of Terrorism Act (PTA), which the new Bill will replace - to cordon off an area and conduct searches within it in their hunt for the perpetrator of the London nail-bombings in Brick Lane, Brixton and Soho in the spring.

In some respects, moreover, the new Bill has modernised and even liberalised the previous legislation. The police will have to seek judicial authority to sanction the longer periods of detention - now to be up to seven days before trial - allowed for suspected terrorists. Under the PTA, the power was, undesirably, left to the Home Secretary. As a result of this change, the UK will no longer need its much-criticised derogation from Article V (3) of the European Convention of Human Rights.

This spells the welcome abolition of a dark corner in the British state's apparatus of law enforcement, as does ending the infamous ministerial powers to impose exclusion orders barring Northern Ireland residents from entering Great Britain.

The definition of terrorism as the use or threat of "serious" violence also rightly creates a higher threshold than the PTA, which referred only to "violence". Nevertheless, the fact that the new definition refers to serious violence against "property" as well as against "persons" has given rise to real worries among environmental and other groups that the powers available under the new Act - including the enhanced powers of arrest, asset seizure, stop and search, and detention - could be used against protesters who constitute no threat to human life.

It is one thing to categorise as terrorism a bombing that destroys a barn without casualties, if it might have threatened a human life. But it is quite another to use the blunderbuss of counter-terrorism powers against, say, those suspected of destroying fields of GM crops. It is unlikely that Mr Straw would personally sanction using the enhanced powers to hunt down Lord Melchett and his Greenpeace colleagues. But a future Home Secretary might. The Bill contains inadequate protection for protesters who infuriate and obstruct, but do not endanger, their fellow-citizens.

The second serious worry concerns the new offence of inciting another person to commit an act of terrorism outside the UK. Though much more tightly drawn than the Tory Private Member's Bill on the subject before the last general election, there are real fears that it could turn into a "Mandela clause". By covering incitement to commit a series of offences - including endangering life by damaging property - it could, at least in theory, threaten residents of the UK who are allied to those fighting a just war against a brutal and repressive regime. True, a prosecution would have to be authorised by the DPP. But, again, that is not an absolute guarantee against authorities in the future moving against highly moral dissidents, under commercial or diplomatic pressure from just such a regime. To be fair to Mr Straw, it will not be easy to redraft these clauses to preclude misuse. But it is up to Parliament to ensure that he does so.