Downing Street's assertion that yesterday was business as usual for the Prime Minister adds to the gaiety of the nation. Business as usual, indeed. For the second time in less than two years, the Labour Party is the subject of a criminal investigation into its funding practices.

These funding scandals may in truth have their origins in the Blair premiership, but that will not help the Prime Minister. The most obvious difference between Gordon Brown's administration and that of Tony Blair is that Downing Street is now liberally sprinkled with banana skins, on which its new incumbent is forever slipping. Mr Brown the careful custodian of the national wealth is a figure of the past; he has been reborn as the Calamity Jane of British politics.

The proxy cheques scandal began back in 2003, as Jack Straw was at some pains to point out yesterday; but some of the biggest cheques landed after Mr Brown entered No 10. The more Mr Brown imitates Pontius Pilate, the more willing he appears to seize the occasion to settle old scores with Harriet Harman and to be generally ready to make scapegoats of his colleagues, the more sceptical the public will be that he intends to root out the culture of lax accountability that is responsible for this mess. People will conclude that instead of a stiff new Scottish besom, they are being treated to a Stalinist airbrushing away of inconvenient truths.

The cash-for-honours business is by no means history. After 19 frustrating and frustrated months, the police closed their sometimes bizarre investigation without bringing charges, putting the Labour Party technically in the clear, but without clearing the air. The lingering impression of sharp practice blighted Mr Blair's last year in office, bringing full circle a premiership that began with the small matter of an unorthodox £1 million donation proffered by the Formula One tycoon Bernie Ecclestone.

Cash-for-peerages was what Sherlock Holmes would have called a two-pipe problem, full of grey legal areas. The Case of the Proxy Donations is not. The law on declaring donations is clearly laid down in the 2000 Act and the paper trail should not be difficult to follow. The politicians that the Metropolitan Police will wish to interview divide into two main camps: the “99.9 per cent of people in the Labour Party” who, Mr Straw insists, knew “nothing at all” — a band which includes senior party officials who insist that they had never heard of David Abrahams, let alone those who signed conspicuously ample cheques to the party on his behalf — and those few, only two so far, who admit knowing about the arrangement, but not that it was illegal.

The police should not allow themselves to be over-impressed by protestations of ignorance. Their little list will presumably include: Labour's just-departed general secretary, its treasurer, its deputy leader cum party chairman, the head of its National Executive Committee, the Prime Minister's campaign co-ordinator and his electoral fundraiser — and, presumably, the admirably well-informed Baroness Jay of Paddington and the other five members of the committee set up in 2002 to vet all major donations to the party. Ex officio, each and every one of these persons had a duty to be thoroughly familiar with the law on party finances. It was, further, their business to know their six-figure donors; such knowledge is the alpha and omega of successful and responsible fundraising.

The Electoral Commission records that three out of five people no longer believe that party financing is open and above board. Yet the irony is that while funding is a mess, British politics is basically clean, and people sense that this is so. That is why the frontbench descent into back- biting may hurt the Prime Minister even more than this further evidence that Labour cannot or will not put its funding house in order. Disarray within a political party is fatal. Mr Brown has never played “broad tent” politics, relying instead on the muscle of his small, closeknit faction to win arguments, control policies and grab the credit. His instinct in a corner is not to beckon allies to his side. But if he continues to lay about him, he will make bad very much worse.