Mr Green's arrest is an affront to democracy: Michael Martin must go

The arrest of Damian Green on a dragnet conspiracy charge and the invasion of his office in the Palace of Westminster demands immediate action, not just a long-distance inquiry.

When Parliament returns this week, MPs of all parties must unite to restore both their own liberties and the rule of law.

No MP is immune from prosecution: insurance fraud (John Stonehouse), spying (Will Owen) and perjury (Jonathan Aitken) are examples of criminal charges that have been brought against less-than-honourable members.

Equally, there can be no objection to a government department - especially the Home Office - with its crime and security concerns, calling in police to investigate disloyal employees who may be "leaking", even to MPs, for corrupt, selfish or mercenary motives.

But police investigations must in such cases be conducted strictly in accordance with the law and with constitutional conventions that require respect for an MP's public duty to receive confidential, even secret, information.

This was settled by Parliament in 1938, when Duncan Sandys MP was threatened with prosecution during Chamberlain's government for obtaining and then divulging military secrets about the lack of defence preparation for war.

A select committee criticised the Attorney-General for preventing Sandys from doing his duty. Later, in 1970, the Attorney-General refused to prosecute Sir Hugh Fraser MP, despite his demand to be placed in the dock, when Official Secrets Act charges were brought against others who had obtained for him secret documents about the Biafran war.

If Speaker Michael Martin authorised the raid on Green's parliamentary offices - and it should only have been ordered by the Speaker - he betrayed our constitutional history.

On 3 January 1642, Charles I and 400 armed troops entered Parliament to demand the surrender of Pym and Hampden, only to be told by Speaker Lenthall: "I have neither eyes to see nor tongue to speak in this place, but as the House is pleased to direct me whose servant I am here and humbly beg your Majesty's pardon that I cannot give any answer other than this."

Speaker Martin should not have let the police in unless he had assurance from the Attorney-General or the DPP that this was a case of necessity and that evidence of serious criminality would be found in Green's office and might otherwise disappear.

Lenthall's defiance of the King is the foundation of parliamentarians' rights to meet and do their public duty without molestation. If Speaker Martin did indeed permit the police to ransack Green's office, he should resign - or Parliament should remove him from office. Otherwise, his decision will stand as a precedent for the harassment of MPs in the future.

Who signed the search warrant? Did the police seek permission from a magistrate as they should have done, or did they authorise themselves under anti-terror provisions - which would amount to a serious circumvention of due process?

Ironically, if the offices sought to be raided in a leak inquiry were those of a journalist or a newspaper editor, the police would have to apply to a circuit judge - an extra safeguard for public watchdogs. MPs forgot to extend the same safeguard to themselves when they passed the Police & Criminal Evidence Act back in 1984: they should swiftly amend it so as to protect their own sources of information.

The first principle of the rule of criminal law is that it must be "prescribed" - ie clear and well established. Why, then, was Green searched and arrested for "conspiracy to commit misconduct in public office", an offence which is hopelessly lacking in clarity and never before used against an MP?

It has occasionally been charged against police officers who fail in their duty to protect civilians and it can also serve to indict police and civil servants who supply confidential information to criminals. But a conspiracy charge based on this judge-made offence (maximum sentence - life imprisonment) is not a satisfactory basis for searching or arresting an MP who has received confidential information.

If Green is alleged to have paid for it, then he should have been charged with specific bribery or corruption offences; if it is alleged that he induced a Crown servant to remove documents, then he can be charged with theft. If he has procured and published sensitive information about security or details relevant to fighting crime, he can be charged under Section 5 of the Official Secrets Act (1989). That would entitle him to a public interest defence.

If, however, the police evidence goes no further than to show that Green - like any opposition MP, journalist or editor - was merely the enthusiastic recipient of newsworthy information outside these categories, then he has committed no crime and should receive heavy damages for wrongful search and arrest. Just like John Wilkes, a liberty hero whose victories for freedom should be celebrated in a British bill of rights.

Much of the debate over the police action has missed the point. Police have operational independence, and it would have been wrong for them to have told their plans to the Prime Minister or Home Secretary. But the police must act according to law and obtain the approval of the DPP before beginning any operation against MPs.

This was what went wrong with the "cash for peerages" wild goose chase which cost a fortune and caused a number of people to suffer much-publicised dawn arrests, all on a mistaken legal premise. The police ceased as soon as a leading counsel was brought in to tell them the law - 15 months afterwards. The moral of that waste of public time and money was that every such operation should be sanctioned at the outset by the DPP.

For all the overblown rhetoric about police states, the reality is more like police mistakes. While a full inquiry must await Mr Green's trial - or the dropping of all charges - parliament can and should act immediately to ensure that henceforth, no MP is searched or arrested without the prior approval of the DPP.

Most important, an all-party committee should be established to work out how to replace the Human Rights Act with a proper written constitution, featuring a British bill of rights that sets forth the liberties that were so hard-won and - as last week's events showed - so easily forgotten.

Geoffrey Robertson QC is author of The Tyrannicide Brief (Vintage) and The Levellers - the Putney Debates (Verso).