Leading article: Mr Brown's Cabinet reshuffle begins ahead of time

Jacqui Smith makes her exit from the Home Office before she is pushed

In an extraordinary few hours at Westminster, even by the frenetic standards of this spring, it was hard to keep track yesterday of who was going, who was staying, and whether the timescale was the end of the week (for the expected Cabinet reshuffle) or the next election. But the highest profile departure to be made known, if not actually announced, was that of the Home Secretary, Jacqui Smith. While she apparently hopes to stay on in the Commons – intending to fight for re-election in Redditch – she is resigning as Home Secretary after almost two years in the job.

Ms Smith's decision has many layers of significance. The first is entirely practical. In leaving vacant one of the three great offices of state, she gives the Prime Minister more leeway than he might otherwise have had when planning his reshuffle. If he wants to move the Chancellor, Alistair Darling, this is one destination that would not look too much like a demotion. If not, it still places a high-level position at his disposal. And, of course, it saves Ms Smith the humiliation of being sacked.

In leaving voluntarily, Ms Smith is doing Mr Brown another favour. As the first Cabinet minister and MP to have her expenses "outed" in the media, she has the dubious distinction of blazing the trail on the scandal that has been blowing Parliament apart ever since. What is more, the particular revelations about her claims – with their petty banality (the bath plug), their sleaziness (the "adult" films viewed by her husband) and the bizarre living arrangements (a room at her sister's house declared as her primary residence) – gave other MPs a graphic preview of what they were in for and set the tone for almost everything that has followed.

But Ms Smith could also be said to be doing the country a favour. Over her two years as Home Secretary, she has proved a disappointment. At the beginning, her appointment looked like a master stroke. By naming a woman to head the Home Office for the first time, Mr Brown deftly allayed fears that women would take a backseat in his government. And she was rightly praised for the calm and quiet way in which she dealt with the failed terrorist attacks in London and Glasgow. By condemning terrorism pure and simple and refusing to talk about "Islamic terrorism", she also reinforced the welcome change of vocabulary initiated by Gordon Brown in eschewing the term "war on terror". This, though, has to be seen as the high point of Ms Smith's career.

Thereafter she proved all too amenable to toeing the Government's repressive line on surveillance, ID cards and the like, while showing no special competence at running her department – a department half its original size after the creation of the Ministry of Justice. It was left to the newly elected mayor of London, Boris Johnson, to show her how to deal with the gaffe-prone Metropolitan Police Commissioner, Sir Ian Blair.

Some may be tempted to see in the fate of Ms Smith, and the impending departure of what might seem a disproportionate number of female MPs – Beverley Hughes and Patricia Hewitt added their names to the list yesterday – as a warning to aspiring women politicians. But this would be wrong. Each has her own reasons for going; alas, Ms Smith was not up to the job.

By putting her job into play now, however, Ms Smith has underlined the opportunity, short-lived though it will be, that Mr Brown must grasp if he wants to fight on. His next Cabinet must combine innovation, competence and, above all, hands that are squeaky clean. His past performance as a talent-spotter does not raise enormous hope.