A damning charge-sheet against the Prime Minister

The defection to the Liberal Democrats of Brian Sedgemore, outgoing MP for Hackney South & Shoreditch, is a signal moment in British politics. Here is a politician of solid Labour stock, not a New Labourite to be sure but no unreconstructed Old Labourite either, who has decided that the time has arrived for him to part company with the party - his party - for good. He gives many reasons, but they boil down to one: the Government's betrayal, as he sees it, of everything the Labour Party once held dear.

The defection to the Liberal Democrats of Brian Sedgemore, outgoing MP for Hackney South & Shoreditch, is a signal moment in British politics. Here is a politician of solid Labour stock, not a New Labourite to be sure but no unreconstructed Old Labourite either, who has decided that the time has arrived for him to part company with the party - his party - for good. He gives many reasons, but they boil down to one: the Government's betrayal, as he sees it, of everything the Labour Party once held dear.

Mr Sedgemore made plain his distress in his last Commons speech during the debate on the Terrorism Bill. That debate he described as a grim reminder of how the Prime Minister and the Home Secretary were giving up on values - to wit: liberty and the rule of law. And he deplored the way Parliament, including so many Labour MPs, had proved so compliant. They had, he said, voted for the abolition of jury trial in some cases; for an unlawful war, for a "gulag" at Belmarsh and for locking up innocent people in their homes. This was, he said, "new Labour's descent into Hell".

He has now decided to salve his conscience, not just by leaving the Labour Party and plighting his troth to the Liberal Democrats, but by timing his departure for the very point when it stands to hurt Labour and the Prime Minister the most. By coincidence or design, it is also the point at which the Iraq war has finally made its entrance into the election campaign. With the weekend disclosure of the Attorney General's ambivalence about the war's legality, the Prime Minister is once again being called upon to defend the decisions he took.

Mr Blair's preference yesterday was to persist in the - highly misleading - defence he offered in his interview with The Independent last week. This argument is that, with 250,000 soldiers massed in the Middle East, Saddam Hussein still defying UN resolutions and a UN Security Council disinclined to approve a war, the only responsible choice was to order the troops into action. He seems to believe that the more he repeats this, the more credible it will be. What he omits to say is that he only faced this choice because of the fateful misjudgements he had made in the weeks and months before.

If any voters are still seeking clarity about the real accusations against Mr Blair, the charge-sheet offered by Mr Sedgemore is not a bad place to start - a better place, certainly, than the Foreign Secretary's interview with the BBC's Today programme, which only muddied already turgid waters. Where Jack Straw's defence relied on obfuscation, Mr Sedgemore's indictment is crystal clear: "contempt for Parliament, contempt for Cabinet government, contempt for the rule of law and contempt for due process, including Habeas Corpus".

Mr Blair and Labour campaign managers will doubtless try to dismiss the charges by minimising the significance of Mr Sedgemore's defection. They will say that, as an outgoing MP, he is scant loss to the party. They will say that, in leaving now, he has shown his true, disloyal, colours. It is also a reasonable bet that aspersions will be cast on his character: those whom this Government sets out to discredit - such as the weapons scientist, Dr David Kelly - they first subject to anonymous character assassination.

Brian Sedgemore says that other outgoing MPs are preparing to announce that they are abandoning Labour, but only after the election. In a political culture where resignation has gone out of fashion, an election gives voters their one opportunity to pass their verdict on a Government. In making his case publicly now, rather than waiting until the votes have been counted, Mr Sedgemore has chosen the more honourable course.