James Purnell, the lone assassin

As assassination plots go, James Purnell's move to finish off Gordon Brown must rank among the most dramatic in recent times - partly because he managed to keep it to himself, and partly because it was so direct and unequivocal.

While the Prime Minister was undoubtedly biting his fingernails to the quick in expectation that the local and Euro election results would prompt a move to oust him at some point over the next few days, he did not spot arch-Blairite Purnell creeping towards him in the undergrowth.

The Work and Pensions Secretary did not wait for the results of the elections before he broke cover and plunged the knife deep into Gordon Brown's front. The very second the polling stations closed he announced he was quitting the Cabinet, called on Brown to stand down for the good of the party and country, and declared he was not seeking the leadership himself.

The last, of course, is a clever, calculated attempt to suggest his actions are motivated purely by a desire to save the country and the Labour party and nothing to do with personal ambition. He did not, of course, say he would not be ready to serve if the call came.

Neither did his statement rule out the possibility that he sees himself in one of the top Cabinet jobs under the next leader, should it be someone from his own Blairite wing of the party like Foreign Secretary David Miliband.

And that is one of the key questions now being asked in the immediate aftermath of his manoeuvre. His statement is certainly the clearest possible challenge to other Cabinet ministers and Labour MPs to now stand up and be counted, and that can only be seriously bad news for Brown.

But does it also disguise the fact that Purnell intends to run for the job himself, or that he has already done some sort of deal with another figure to allow that person to emerge as a leadership challenger with clean hands. And, if so, who?

David Miliband has denied it's him and has joined other senior ministers, including Defence Secretary John Hutton, in rallying behind Brown and apparently distancing themselves from Purnell's plot. But few doubt Miliband would be a contender for the leadership should the latest developments lead to a challenge to Brown.

Following his suicidal behaviour last autumn when he appeared to be manoeuvring to oust Brown, Miliband has been keeping his head down and trying to rebuild his credibility. But any suggestion there was a plot by the Blairites to seize control of the government and party might just backfire. And, of course, Purnell and Miliband deny it all.

However, despite all the signs Purnell's action was a solo effort and not part of a wider Blairite conspiracy, the impression that Blair's friends are on the march has already started to take hold in some quarters, with all the usual suspects - Charles Clarke, Alan Milburn, Stephen Byers and now resigned minister Hazel Blears  - firmly in the frame. 

Even MP Barry Sheerman's call for a secret ballot of Labour MPs on Brown's leadership, issued yesterday but quickly overshadowed by Purnell's announcement, is being seen by some as part of the same plot.

For many, then, Alan Johnson remains the most likely figure to step into Brown's shoes. He is not tainted by too close an association with any faction and is believed to offer the best hope of uniting Labour and averting a possible outbreak of civil war between the Blairites and the Brownites, who may still be looking towards Ed Balls as their candidate.

And the other big question remains - how will the coup de grace actually be delivered? Will it see a queue of senior minister, led by a figure such as Chancellor Alistair Darling, telling the Prime Minister he is finished, or will it see a stalking horse candidate such as Charles Clarke announcing a leadership bid backed by the now infamous email letter demanding Brown's resignation.

Meanwhile, Purnell's move has made the results of the local and Euro polls virtually irrelevant - unless they throw Brown an unexpected lifeline by being far less disastrous than expected - and the Prime Minister has to complete a reshuffle with options to reassert his authority severely limited.

It is now likely that the reshuffle will come in the next few hours. It is Brown’s last chance has to inject some sense of political momentum into his team. Recent events, however, suggest he may well be seeking to turn back the tide. ·