It was supposed to be a new era. When he accepted the Labour leadership and the premiership of this country two years ago, Gordon Brown promised “a different kind of politics”, one that was more open and honest. He would be guided, he said, by the “moral compass” that had been provided by years of listening to his father’s sermons.

Just in case anybody did not get the message, Harriet Harman, his deputy, filled in the gaps. “In future, under a Gordon Brown regime, we need to have no spin, no briefing, no secrets and respect for parliament,” she promised. Blair-era tactics would be consigned to history in this new era of honesty.

Now we get the reality. A dirty tricks operation has been operating inside Downing Street with Damian McBride, Mr Brown’s former spin doctor, at the heart of it. Mr McBride, who was in charge of “strategy and planning” and earned the sobriquet “McPoison” when dealing directly with journalists, has been living up to his new name.

Today we reveal that he intended to drip poison about senior Tories into the public domain via a Labour-sponsored political web-site called Red Rag. It included entirely unsubstantiated smears about the private life of George Osborne, the shadow chancellor, and his wife. It suggested that embarrassing disclosures could be made about David Cameron’s medical history. Any bit of rumour or tittle-tattle was apparently fair game for Mr McBride, the more embarrassing the better.

The prime minister, having promised a new era, is knee deep in the worst aspects of new Labour spin. The recipient of the McBride e-mails setting out these allegations was Derek “Dolly” Draper, once Lord Mandelson’s right-hand man, who became embroiled in the “lobbygate” scandal over access to ministers by political lobbyists soon after Labour took office. Copied in to at least one of the e-mails was Charlie Whelan, Mr Brown’s notorious former spin doctor, whose resignation was forced by Tony Blair as long ago as 1998 over the Mandelson home loan affair.

The e-mails showing how the prime minister’s head of strategy and planning was intending to spread dirt on the Tories date from January. This was the time when tens of billions of pounds of taxpayers’ money was being handed out in a desperate bid to prop up the banking system. The very next day the government announced a £20 billion plan. It was when, according to Mr Brown, he and the entire government’s attention was focused on solving the banking crisis and dragging the economy out of its worst recession in the postwar era.

Now we know it was not. One of the prime minister’s most senior lieutenants, whose salary came courtesy of taxpayers and who composed the offending e-mails from his Downing Street computer, was spending his time researching ideas, in his words, “intended to destabilise the Tories”.

This was a disgraceful abuse of public money and of a public position. Why should we have been paying the salary of somebody whose idea of “strategy and planning” is to spread scurrilous rumours about opposition politicians? What kind of operation is Mr Brown presiding over in Downing Street?

It was never enough for Mr McBride to apologise, as he tried to do in advance of the publication of the e-mails. So last night, belatedly, Downing Street announced his resignation. He will not be missed. But the culture in which he operated survives him. If the prime minister wants to remove the poison from British politics, he has to ensure he has eradicated it from Downing Street.