Keith Ewing (King's College London):  According to Vernon Bogdanor, the Damien Green affair is a ‘storm in a teacup’; while according to Harriet Harman it raises ‘big constitutional principles’. They cannot both be right.  What also cannot be right is the Home Office response that this is an operational policing matter, which is not the responsibility of ministers (who as Jackie Ashley points out in this morning’s Guardian are first and foremost MPs).

Although the facts and circumstances of the affair continue to unfold, and although we do not yet have a clear picture of what has happened, there are nevertheless a few inconvenient principles around which the debate needs to be constructed, principles some of which to his great credit Dennis MacShane appeared to identify well on the Today programme, also this morning.

Without commenting on specific allegations, civil servants who leak may (or may not) be doing so for noble purposes; but they also know what the constitutional and personal consequences will be. Members of Parliament who put leaked material into the public domain may be performing a valuable public service; but there are lines even they should not cross.

Members of Parliament have no general immunity from criminal liability; but the police nevertheless should behave with judgement and restraint. Quite whether it was necessary to arrest Damien Green MP in this case, to search his home and his constituency office, and to do so with a small army of rubber-gloved Special Branch officers, will no doubt be revealed in due course.    

But above all of this is the sanctity of Parliament. Whatever has happened in this case, it is not yet clear that anything took place that would begin to justify the police entering the Palace of Westminster, with or without a warrant, to search the office of a Member of Parliament, and possibly examine his working materials, with information dealing with potentially dozens of cases involving hundreds of constituents.    

The police were not – in this case - looking for dead bodies, murder weapons, or kidnap victims. There was no evidence (that we are aware of) of another gunpowder plot, which might justify the invasion of Parliament in extremis. And there was no posse of Countryside Alliance protestors about to chase a fox onto the floor of the Commons; or climate change protestors about to parachute from the roof.

As events continue to unfold around this compelling drama, there are hard questions for those who issued the warrant – why was it thought appropriate that a warrant should be granted to search the parliamentary offices of an MP? And hard questions too for the Commons authorities - faced with police officers armed with a warrant, is it their view that the police should always be allowed to pass?           

In the typically British way, however, the temptation will be to manage an outcome without any of the key questions being properly resolved. But what ought to happen is that the matter should be referred later in the week to the Commons Standards and Privileges Committee - for an explanation from those who trespassed on the authority of Parliament, and to establish clear rules about when the police can enter Parliament.

That investigation will discover that this is not the first time that the issue of search warrants has given rise to difficulty, with similar problems having arisen in Commonwealth Parliaments, where it has been revealed (notably in New South Wales) that ‘the execution of search warrants, the issuing of subpoenas and orders for discovery process remain areas of uncertainty in the law of parliamentary privilege’.

In the longer term, the case reveals a need to address what appears to be a real flaw in the protection that parliamentarians ought to demand. If the rules of parliamentary privilege (and those who are supposed to protect them) cannot safeguard parliamentary offices from this kind of invasion, then MPs should demand the same statutory protection that is provided to lawyers for confidential legal materials.

The latter cannot be obtained willy-nilly by search warrants. Failing such protection, MPs should at least demand the same protection enjoyed by journalists and doctors whose confidential materials can – as a general rule  - only be ordered up after an inter partes hearing before a judge, at which some of the big constitutional issues to which Harriet Harman referred could be fully considered.     

Labour MPs inclined to brush all this off with a shrug and smile should fast forward in their minds to the next Tory government. Imagine that a leading front-bench spokesman is arrested, detained and questioned by the police for nine hours, has his home and constituency office searched by nine rubber-gloved police officers, and then has his Commons office searched. It could be you. Storm in a tea cup?