Graphic: love-hate relationships in the Cabinet | Comment Central: Everything you need to know about Johnson

The bookmakers’ strong favourite to succeed Gordon Brown will be watching and waiting this weekend, wondering whether the one chance that he will ever have of leading Labour will come his way.

Alan Johnson’s face was, in the words of one observant Conservative, an “impassive mask” as the drama of Prime Minister’s Questions was played out yesterday, with Labour MPs trying to rally to their beleaguered leader.

Mr Johnson cannot afford one step out of place. Like other members of the Cabinet the Health Secretary will continue to voice his loyalty to Mr Brown, and it will be genuinely expressed.

As rumours of plots swirled around Westminster, Mr Johnson backed Mr Brown. He told the BBC: “He is doing the job and there is absolutely no one who could do that job better.”

He said that Mr Brown’s performance at Prime Minister’s Questions had shown he had the “courage and tenacity” to lead the country “under circumstances made difficult for him”.

But Mr Johnson also knows that events have conspired to make him a certain candidate in the eyes of Labour MPs if a vacancy were to arise.

And those who know him best believe that this time, unlike in 2007 when he said he did not feel that he was up to the job and did not stand for it, he would take the opportunity.

Mr Johnson has obvious qualities for leading Labour out of its crisis. He is not a divisive figure, never having attached himself to the Brownite or Blairite wings, but he is a man whose obvious working-class credentials would suddenly present the Conservatives with a problem.

Tory MPs admit privately that Mr Johnson is the man they most fear as they believe that his down-to-earth skills can be set against the sense of privilege associated with their own front bench.

His defeat by Harriet Harman in in the contest to be deputy party leader two years ago was a surprise, with some of his supporters saying that he had not worked hard enough. Friends say that he learnt a lot and will not make the same mistake again.

But Labour MPs know that if they ditch Mr Brown and go for Mr Johnson, or anyone else, they will almost certainly have a general election to fight within weeks. It is accepted at Westminster that Labour could not, for the second time in a Parliament, get away with choosing a new leader without an election soon afterwards.

That probably explains why Mr Johnson is the favourite. Most MPs believe that the public will take to him, with his easy charm being a welcome change from Mr Brown’s stern demeanour.

But for other ministers in the Cabinet, or on its edge, it will not be a easy weekend. They all know that it would not take many more of them to go for Mr Brown to be pushed off the cliff.

This time last year, when Mr Brown was again in trouble, all eyes were on David Miliband and James Purnell, the Cabinet’s leading Blairites. Mr Miliband’s attempt to set out his stall went badly wrong. But they will be watched again.

So, too, will Caroline Flint, the Minister for Europe, who is a friend of Ms Blears and praised her this week. She also attended the Commons speech on Tuesday of Jacqui Smith, soon after it emerged that she was going.

As Mr Brown carries out his reshuffle between now and Monday, the figure who may carry the key to his future is Alistair Darling. Those who know Mr Brown’s mind say that he still wants to put Ed Balls in the Treasury.

It is suggested that he has told Mr Darling so and given him a choice of other jobs. But if the Chancellor digs in his heels, it is hard to see how Mr Brown can force him out. He can survive a Hazel Blears. It is unlikely he could survive an unhappy ex-Chancellor.

Times Recommends

• Oh no, Darling: you've failed
• BBC survives dogfight over Kennel Club expose
• Where the Rosetta belongs can’t be set in stone