Acting with the stealth and speed of a man running out of options and time, Brown accelerated the announcement of his new line-up to lock in any other cabinet doubters before the full extent of Labour’s crushing defeat in Thursday’s local elections became known.

Brown’s strategy for political survival rested on three crucial cabinet choices. Most importantly, Alan Johnson, the 59-year-old former union leader who is favoured by many in Labour to succeed him, was promoted to the top post of Home Secretary, from his previous post as Health Minister.

Two other cabinet heavyweights whose positions were in doubt were reconfirmed in their jobs, Alistair Darling as Chancellor of the Exchequer and David Miliband as Foreign Secretary.

Other promotions and shifts seemed to be a matter of political housekeeping, and awaited formal confirmation in a late afternoon news conference scheduled by Brown. The early morning cabinet shake-up was so hurried that details were made known by Brown’s aides, and confirmed in broadcast interviews by some of those involved, instead of the normal pattern of making the changes known in a formal, written statement.

The reshuffle’s urgency was heightened on Thursday night when James Purnell, the 39-year-old work and Pensions Minister, became the third cabinet minister to resign in 72 hours, after Home Secretary Jacqui Smith and Communities Secretary Hazel Blears. But unlike Smith and Blears, both of whom were caught in the parliamentary expenses scandal that has overwhelmed British politics in the past month, Purnell used his resignation to issue a demand for Brown to step down.

In his resignation letter he demanded that the PM quit “to give our party a fighting chance of winning” a general election that must be held before Labour’s five-year parliamentary mandate expires next June.