Leading article: Our freedoms under threat

There is no law, no system, no set of regulations which can more effectively hold governments to account than the conscience of man. Opposition parties, the public and the press rely on individuals, not systems, to tell us what those who rule over us would like us not to know. We call them "whistleblowers" because, like referees, they seek to keep the players in our political system in check.

Prime Minister Gordon Brown knows very well the importance of whistleblowers - his preparations for the 1996 budget debate were greatly assisted by Labour's obtaining of a copy of the document the night before. It is hoped, therefore, that if the Metropolitan Police have no case to make against shadow Immigration Minister Damian Green beyond that he used a whistleblower to bring accurate and important information into the public domain, Mr Brown will be among the first to condemn the arrest of a parliamentary colleague by the anti-terror squad.

Mr Green was interviewed for nine hours; his mobile phone and computer were seized by police, and his house, Commons and constituency offices were searched by nearly two dozen counterterrorist police, though as far as is known, their operation has nothing whatsoever to do with "terror" issues. Rather, the Met investigation centres around a junior Home Office official accused of regularly leaking memos to Mr Green, exposing serious errors by his department which included the clearing of thousands of illegal immigrants for work in sensitive Whitehall security jobs.

Crucial details about the case are still not yet known: the full extent of Mr Green's relationship with the civil servant and the full details of the information he obtained. But at first sight the Met would seem to have badly miscalculated and badly mistreated Mr Green in the process.

Following the 1996 budget debacle, leaks to Labour from 10 and 11 Downing Street became so bad than a full-scale internal investigation was launched into the issue. At no point, however, were members of the Opposition arrested by the terror squad. Had that happened, Labour members would have screamed with justifiable outrage. Now a government department eager to introduce ID cards and detention without trial, Taser guns and citizen databases, is once again exposed to the accusation that the powers and systems it has given, and seeks to give, the police are observably undermining our freedom.

As for the Met, whose commissioner, Sir Ian Blair, stepped down yesterday, they too must consider their reputation which has been damaged by numerous botched operations in recent years, not least regarding politicians and their party donors. The next commissioner must work hard to restore our confidence in the force and avoid political scandals: there were too many during Sir Ian Blair's days in command – including, as it turns out, on the final one.