The invitations have already been sent out for Harriet Harman’s “Christmas celebration” on Wednesday. For £30, guests are promised champagne, canapés and comedy.

It is a fair bet that Gordon Brown will find that he is unable to attend the event at the Sound nightclub in Leicester Square - or, indeed, any other fundraising effort that Labour’s deputy leader might arrange in the future.

The Prime Minister and Ms Harman will present a united front today when they both address Labour activists at a policymaking meeting. Behind the scene No 10 aides are furious at the Harman camp’s attempts to pass the blame on to Mr Brown for her involvement in Labour’s donations scandal. Mr Brown had his competence and integrity called into question by David Cameron at Prime Minister’s Questions over his admission that a controversial property developer used proxies to channel money to Labour. Now, as the cracks begin to show after six days of pummelling over the David Abrahams affair, the Tories can claim that Mr Brown’s administration is not only bungling and sleazy, but divided to boot.

The mood in No 10 on Monday morning was one of bewilderment as news began to filter through that Peter Watt, Labour’s general secretary, had admitted to Mr Brown that he knew Mr Abrahams was channelling cash to the party through third parties. “I can’t believe this is happening,” one figure admitted as the details emerged of how more than £600,000 was sent through four intermediaries over four years.

By the next day Mr Brown’s anger had become “volcanic” as Ms Harman struggled to answer how she came to accept a donation for £5,000 from one of the proxies towards her deputy leadership campaign. In particular, the Leader of the House conspicuously avoided saying whether her team had solicited the cash from Janet Kidd, a secretary in Newcastle upon Tyne.

Even Labour MPs minded to give Ms Harman the benefit of the doubt questioned her failure to check the donor’s background – particularly since her husband, Jack Dromey, is Labour’s treasurer.

Calling his inner circle to a meeting on Tuesday night, the Prime Minister left them in no doubt that anyone found guilty of the slightest misdemeanour over party funding would not be spared. But when just a few hours later Jon Mendelsohn, his chief fundraiser, admitted that he, too, knew about the arrangement, Mr Brown’s instincts were to believe the man that he brought in to restore trust in Labour’s donations after Lord Levy.

Preparing for PMQs - with Geoff Hoon, the Chief Whip playing the role of Mr Cameron - Mr Brown rehearsed his defence. A statement from Mr Mandelsohn insisting that he had been assured by Mr Watt that the arrangement was legal and that attempts were being made to end it in any case, was released moments before noon to give the Conservative leader as little time as possible to react.

In addition, Mr Brown had announced a series of new inquries, the most urgent headed by Lord Whitty, the universally respected former Labour general secretary. The contrast with Tony Blair’s foot-dragging over cash for honours was intended to be marked.

For a time it appeared that Mr Brown might be getting back on the front foot. His references to funding scandals in the Conservatives and Liberal Democrats united Labour back-benchers who had endured torrid abuse from their opponents all week. That afternoon, unnoticed, Lord Whitty slipped into Labour’s Victoria Street HQ, fulfilling a promise to Mr Brown to carry out his investigation with maximum speed.

Almost immediately he turned up yet another embarrassing disclosure - and this time one uncomfortably close to Mr Brown. Chris Leslie, the former MP who acted as Mr Brown’s campaign co-ordinator in his unchallenged bid for the leadership, admitted that he, too, had been approached by Mr Abrahams.

According to his later statement, Mr Leslie insists that the property developer said that his secretary, Janet Kidd, wanted to make a £5,000 donation to the campaign. Mr Brown’s campaign – not short of funds, rejected the donation when it arrived. When, months later, a member of Ms Harman’s team asked Mr Leslie for suggestions of possible donors, Mr Leslie remembered Mrs Kidd and passed on her details.

Suddenly Ms Harman’s reticence over whether the donation had been solicited made sense. Far from seeking to hide her own culpability in the affair she had been constrained from using her best defence since it implicated Mr Brown. The reason she had failed to check Mrs Kidd’s bona fides was because the donor had been recommended by the Prime Minister’s chief campaign aide. It also helped to explain why Mr Brown had shifted to protect the deputy leader he had failed to give full backing earlier in the week.

With both their fates entwined, No 10 and Ms Harman spent most of Thursday agreeing how best to come clean. The final plan was settled mid-afternoon - a statement from Mr Leslie would be given to the Press Association, followed by another from Ms Harman’s campaign.

With the sort of ill-luck scandal attracts, the second statement was given far greater prominence when it was put out by PA while the first went almost unnoticed. With newspapers close to deadline, it seemed at first that the Leader of the House had launched an astonishing preemptive attack on Mr Brown.

The impression was reinforced when supporters of Ms Harman contacted journalists, adding the extra detail that Mrs Kidd’s cheque to her campaign was accompanied by a letter stating “as discussed with Mr Leslie”. The detail - which could prove to be a key to the police investigation - undermines attempts to minimise Mr Leslie’s involvement.

What had been intended as a joint operation backfired spectacularly in a welter of “Labour at war” headlines the next morning. Jack Straw, the Justice Secretary, did little to counter the impression of Labour’s house divided when he was asked if the Abrahams scandal was a hangover from Tony Blair’s period of office. It was, he said, a “matter of record” that the arrangement had started under Mr Brown’s predecessor. Mr Straw also seemed to question Mr Mendelsohn’s “state of mind” in not alerting anyone on discovering the Abrahams dodge.

It all makes Ms Harman’s party on Wednesday one of the more unmissable events of the season. It is, as the invitation trills, “the last opportunity to raise funds towards the costs of our very successful campaign”.

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