Mary Dejevsky: We've lost sight of our most fundamental rights

MPs elsewhere commonly enjoy immunity from arrest and/or prosecution

Imagine you were playing one of those word-association games and the word given was Parliament – what would be the first idea to come to mind? In the week of the State Opening, you might offer pageant, privilege and, yes, democracy. All three were generously displayed on Wednesday as the Queen arrived at Westminster in her coach, Black Rod knocked three times only to have the door of the Commons slammed in his face and the elected Government's legislative agenda was read out by the monarch from her throne. How well, you might have remarked – with a sense of relief in these troubled times – we as a nation do this sort of thing. There is still something right with the world.

This year, though, there was a sense in which each of these justified sources of pride had somehow been subverted. The pageant was suddenly form emptied of substance; out of all proportion to the perfunctory and ill-assorted character of the programme. As for privilege and democracy, the still unfinished saga of Damian Green's arrest has exposed in scandalous fashion the tenuous nature of both. The Home Secretary Jacqui Smith's self-righteous statement in the Commons yesterday – in which, according to Mr Green's tart point of order, she appeared misinformed about what exactly he had been arrested for – only reinforced the gathering impression of ministerial confusion.

But the case of Mr Green and the civil servant Christopher Galley, who is suspected of leaking confidential information, is not primarily a matter of clarity or competence – though the lack of both surely enters the equation. There are the highest of principles at stake here, principles that explain why Parliament – far more than the City of London, even in its heyday – still stands as one of the most admired institutions anywhere.

Imagine if something like this had happened elsewhere in the EU, or – for argument's sake – in Russia. As a foreign correspondent, I might have reported it along these lines: "The state police arrived yesterday at the entrance to Parliament and demanded access to the office of an Opposition MP. They proceeded to search the office and remove documents and computer parts in connection with an investigation into leaks of confidential government documents. The parliamentary authorities later conceded that the police had presented no search warrant. The MP was arrested, in front of his distressed family, and his home and other offices were also searched. He has not been charged."

And how would I have interpreted such a sequence of events? Without doubt, as a series of unfriendly, politically inspired acts instigated by a paranoid government, demonstrating the politicisation of the police and the judiciary.

Now there are two reasons why, in fact, these precise circumstances would not be reproduced in many countries of the world. The first is that MPs elsewhere commonly enjoy immunity from arrest and/or prosecution. If the authorities believe they have a case against them, they must first make it to Parliament, and MPs must then vote to lift their colleague's immunity. This is not easy to do. It is a little like extradition; the burden of proof is deliberately heavy: to prevent the government of the day using the police or judiciary against elected representatives of the opposition.

MPs in Britain do not enjoy immunity of this kind, and it is a tribute to the perceived fairness of our institutions, at least hitherto, that there has been no serious call for it. While the judiciary and the police are servants, not of the Government but of the Crown, MPs are subject to the same laws of the land as other citizens – and this can be only a good thing. It is worth noting that Damian Green cheerfully accepts this state of affairs. His objections are not to the principle of an MP being arrested, but to the manner in which it was done, and to the fact that police were given access and permitted to search his office at the House of Commons.

This is also why his fellow Conservatives have been so incensed, and why even many Labour MPs are uncomfortable. The Tories detect a political motive behind Mr Green's arrest; Labour MPs can all too easily envisage themselves in a similar situation in future. But the question they are all asking is the same: how was it that, when the police came knocking at the Commons door, they were not given the Black Rod treatment, or anything like it, but were simply shown in, after the Serjeant-at-Arms had taken – as the police are now telling it – a modicum of legal advice. It is not that the parliamentary authorities put up only feeble resistance (as was clear from the Speaker's abject statement on Wednesday and Ms Smith's more prickly account yesterday), they put up no resistance at all – even though, this must be stressed, they were fully entitled to do so.

The other reason why a close parallel with the Green case would be unlikely elsewhere is the uncertainty and confusion surrounding who ordered whom to do what, and why. Elsewhere, a political motivation would most likely be beyond doubt. Here, the case is clearly shot through with politics: the Government was clearly infuriated by the persistent leaks of sensitive information to the Opposition, and the Cabinet Office called in the police in an effort to stop them. The use of the word "grooming" to describe Mr Green's relationship with the alleged leaker also suggests deliberate innuendo and malice.

But there is also plenty of evidence to support cock-up rather than – or at least, alongside – conspiracy. That the police turned up at the Commons without a search warrant, even though they had obtained warrants to search Mr Green's other premises, suggests ignorance or oversight. The shifting grounds given for the searches, from anti-terrorism to official secrets, suggests a frantic quest for justification. While the Speaker's expression of regret, even as he blamed the less experienced Serjeant-at-Arms, suggested a no less frantic search for scapegoats.

In the end, though, the really shocking aspect is none of this – not even the strong whiff of political motivation. It is the cavalier disregard, or abject ignorance, it exposes of Commons convention among the people charged with its protection. In the moment that the Serjeant-at-Arms signed the police consent form, the centuries of custom and practice that have made the British Parliament what it is were summarily overthrown. At that time and in that place, no one, it seems, had any sense of the constitutional clash that threatens when law-enforcers challenge legislators. If that sense is lost, no amount of pageantry will be able to compensate for it or bring it back. Root and branch modernisation of our whole constitutional order may be the only remedy.