Several important issues have been raised by the arrest of the Conservative immigration spokesman, Damian Green, on suspicion of �conspiring to commit misconduct in a public office�.

Several important issues have been raised by the arrest of the Conservative immigration spokesman, Damian Green, on suspicion of "conspiring to commit misconduct in a public office". He was released without charge, pending the outcome of a police investigation, after officers removed prodigious quantities of documents from his home and his Commons office. Earlier last month, the Home Office civil servant accused of leaking information to him was suspended on full pay.

So far attention has focused on the freedom of MPs to hold the government to account, and whether police breached parliamentary privilege when they searched Mr Green's office. "Brown envelope" carries a certain double entendre for the government because the term does not merely describe the way opposition members often receive politically embarrassing bits of anonymous tittle-tattle from civil servants; it also reminds us that Gordon Brown himself was highly dependent on such leaks when he was on the opposition benches.

The job of opposition is challenging enough at the best of times. The apparent heavy-handedness of the police in this instance has given the Tories a propaganda coup, with senior Labour figures weighing in on their side. "Stalinesque," said David Cameron. "Police state," said Tony Benn. This overstates the case: if these descriptions were true, they would not be at liberty to articulate them. Nevertheless, as Harriet Harman conceded yesterday, there are "serious constitutional principles" at stake and the manner in which the police were allowed free access to Mr Green's office could put Commons Speaker Michael Martin back in the spotlight this week. However, there is a second principle at stake here, too, which is about the government's right to govern. Inevitably, this involves much internal discussion within different departments about the pros and cons of policy options. Sometimes, to test the waters, proposals are selectively leaked to the media, but this government has been the victim of a veritable torrent of leaks clearly not of its making. The police investigation at the Home Office was precipitated not by the Prime Minister or Home Secretary but by the chief civil servant at the Home Office. Another hunt is going on for a mole at the Treasury after the impact of the pre-Budget report was neutralised by the prior leaking of all the main items.

There is a debate to be had about what constitutes the public interest and the line can be a fine one. Clearly, there is a distinction between whistleblowing (especially in the case of something illegal that deserves to be exposed) and a public servant regularly passing on information for the sake of political advantage or financial gain. And there is a difference between a politician receiving occasional unsolicited items of information and actively encouraging a civil servant to breach his contract of employment by passing on confidential information. It is important not to rush to judgment in this case.