Tony Blair, bested in the fight to force Roman Catholic adoption agencies to consider same-sex couples, had a question for the man who had masterminded his defeat. “How come you — Alan Johnson — are a bigger gay icon than I am?”, the then Prime Minister asked.

It is the sort of joke one top-flight politician could make only to another; a dig at their mutual vanity wrapped in an affirmation of a shared self-awareness all too rare in politics.

The trait, an easy charm and relaxed honesty, served Mr Johnson well on his extraordinary rise — until, that is, he was on the threshold of the very top flight of power. Former postman, former union leader, former Education Secretary, Mr Johnson was the last serious candidate for Labour’s leadership to bow before Gordon Brown’s march to Number 10.

But, beaten in the race to be Mr Brown’s deputy, he was undone by honesty when asked why he had not run against Mr Brown in October 2007. “I don’t think I would have been good enough, frankly.”

He was stung, in the aftermath of that interview, to receive letters accusing him of letting down working-class children who aspired to emulate his route to the top. After a period reflecting on that self-assessment, he has quietly been seeking to retract it.

Mr Johnson, who was brought up by his sister after being abandoned by his father and orphaned at the age of 12, does not like to hear these circumstances described as a “wonderful back story” (and is chary of those in Number 10 who want him to play the class card against David Cameron).

After leaving his school without O-levels, Mr Johnson stacked shelves at Tesco’s but walked out because he was not allowed a lunch break. Initially a supporter of the Communist Party of Great Britain, his rise to the head of the Communication Workers’ Union coincided with a drift to the right. By 1994 he was the only union leader to support Tony Blair’s proposal to scrap Clause IV. He came to Mr Blair’s rescue again, as Education Secretary, by forcing through legislation to introduce tuition fees.

He is admired as a wily, tough campaigner, so his defeat by Harriet Harman came as a surprise. One explanation — that he had not shown enough commitment — gathered credibility after a disastrous interview on Desert Island Discs. Having written himself out of the script once, Mr Johnson knows that when the vacancy arises he must this time seize his chance.

Times Recommends

• In Margot Fonteyn's footsteps
• Forget beautiful people, let's have wonky Brits
• Fundamentalist brew: faith and power