Ms Jowell is the networker par excellence, but she is kind and popular with it, a soothing presence wherever she happens to be. Mr Mills can be an altogether more abrasive character, never afraid to voice his opinions.

Stories abound of how at dinner parties Ms Jowell has often had to come to the rescue of an unfortunate guest being harangued by her husband. Friends say that she has always been tolerant and understanding of him.

Last week that tolerance was stretched beyond breaking point and they will now live apart, Ms Jowell at their North London home and Mr Mills in the Warwickshire countryside. While the cynics hint that the move is designed to save her career, their friends say that the suggestion is grotesque given the obvious heartache to them and their family.

As Ms Jowell and Mr Mills began their new existence yesterday, their closest confidants insisted that the decision to go for a “trial separation” was taken jointly; but there is no doubting that Ms Jowell was the wounded party, deeply let down by her husband, and there would be little surprise if she was the instigator.

Ms Jowell’s failure to ask questions of her husband remains her biggest problem as she fights for her political life.

There has been shock in some quarters that Ms Jowell said last week that she knew nothing of the £350,000 received by her husband from Italian sources, but people who know them well are not surprised. In their division of responsibilities, Ms Jowell pursued her political career, while being a devoted mother to her son and daughter, and Mr Mills looked after the finances. As one of the children said, their father’s finances were something of a “closed book”.

That set-up could work only so long as Mr Mills, in his complex, multifarious financial dealings, did nothing that could embarrass his wife in her career.

Last week she discovered that he had apparently not kept to his side of the unspoken bargain. It is believed that the idea of a separation had been considered for some time, but friends say that as Ms Jowell spoke with her permanent secretary as part of the inquiry by the Cabinet Secretary into whether she had broken the ministerial rules, she became more and more distraught as she found that there was so much that she should have been told.

Early last week she realised that Mr Mills had told her nothing of the Italian “gift”. Because Mr Mills regarded it as a present, he had a duty to tell his wife about it so that she could declare it. When The Times asked a confidant how she would be able, after the publication of the Sir Gus O’Donnell investigation, to justify not knowing about the money, the reply was: “He has not told her a bloody thing about it.”

Those words appear to have reflected the growing anger in the Jowell inner circle at the way she was being made to look stupid because of Mr Mills’s behaviour.

Then, just as Ms Jowell was about to be cleared of any wrongdoing by the O’Donnell inquiry, came the devastating news that Mr Mills had used his marriage to her and the name of the Prime Minister in a letter to the Dubai authorities about their refusal to grant him a licence to practise there. “I have the support and sympathy of very many people in public life, from the Prime Minister down,” he wrote.

If there was a final straw, this was it. This had been a traumatic week, but to Ms Jowell the ultimate sin would have been to bring the Prime Minister into the affair. She was furious and beyond consolation.

It was on Thursday night and Friday morning that the pair decided to go their own ways. Mr Blair was told by Ms Jowell on Friday afternoon.

Some in Westminster’s sceptical world will insist that the whole separation idea has been a ploy to save Ms Jowell’s career. To some Labour MPs who are keen to use these events as another example of the moral bankruptcy of the new Labour project, it was here that the hand of Alastair Campbell inevitably appeared, scripting the separation in the way he once asked Robin Cook to choose between his wife and mistress.

The facts do not appear to fit the charge. Mr Campbell has been talking regularly to Ms Jowell and Mr Mills, who are friends, but is adamant that he knew nothing about the separation until Mr Mills told him at the end of the week. Other close friends say it is a travesty to suggest that such a hitherto devoted couple would split in the hope of political gain.

Nonetheless, there is one clear practical objective of the break-up. Ms Jowell can concentrate on her fight to maintain her political career while Mr Mills tries to clear his name of the charges brought against him by the Italian authorities.

As they were finding last week, trying to do both while living in the same household, with the long-held trust of one in the other under growing strain, was proving impossible. A good friend of both said last night: “It was going to drive them both mad. They need time and space to see whether it can be put back together again when this is all over. David wants that. I don’t think Tessa is so sure.”

Last night Ms Jowell was preparing for her appearance in the Commons today. A friend said that she was heartbroken over the break-up. She continues to support her husband in his coming ordeal and maintains that he has done nothing wrong, but she is emotionally exhausted. Her escape over the coming weeks will be to throw herself into her work. She is fighting to survive, but it will be some time before she knows whether her marriage can be rescued.

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