Despite all the column inches recently devoted to pinning down John Prescott's precise class identity, the most significant thing about the whole psychodrama has been overlooked. That is that Prescott's original confession about being "pretty middle class" (and it was a weary kind of statement he made to John Humphrys on Radio 4, not a boast) marked the first instance in the history of the British labour movement of anyone playing up the social scale, not down. That is something the Tories always did, Ted Heath being the best example, and to understand why Prescott discarded his flat cap is to reveal an important, if subtle, prejudice at work in the culture of new Labour.

Labour leaders from the pit and mill always used to hold on to their proletarian identity, even if they hadn't actually handled coal or cotton for half a lifetime. Keir Hardie called himself a miner until the day he died; Ernest Bevin may have been foreign secretary, but he didn't stop being a general labourer in his - or anybody else's - mind. In those days, to decommission your working-class status was to throw away one of your best political weapons.

The fashion was to go the other way - to invent, or at least inflate, a working-class background. Sometimes it was done on your behalf. Tony Crosland, just down from Oxford, sat through his first selection meeting, listening to the secretary of the local Labour Party explain how the newcomer divided his days between factory and night school. "I must stop this," he wrote in his diary, "before it goes too far."

Harold Wilson was always trying to make his roots more humble than they actually were. When reporters were around, he would ruin his food by soaking it in vinegar and HP sauce. During the battle for the succession to Hugh Gaitskell, the Yorkshire accent thickened and the pipe was more assiduously sucked (in private, Wilson favoured the much more socially incriminating cigar). His fellow contenders in the leadership race were the rather more proletarian figures of George Brown and James Callaghan, and Wilson needed to compete in the Horatio Alger stakes. That he didn't try something more ambitious - claim rickets as a child, say - was probably due to a previous botched attempt to dress up, or rather dress down, his past. In 1948, as a young cabinet minister, Wilson made his infamous "barefoot speech", in which he claimed that half his school chums in Huddersfield had been shoeless. Egged on by the press, the whole of Yorkshire seemed to want to testify that this was not - could not - be true. It was an important lesson for Wilson. In future, he altered his style, not his biography. This is what most of his colleagues did when they wanted to strike a proletarian chord. Hugh Dalton ate peas off his knife, Michael Foot wore a donkey jacket, Tony Benn drank pint mugs of tea.

But the case of Michael Meacher, now environment minister, throws the sharpest light of all on the Prescott quandary. In the late 1980s, when the Labour Party was still old, Meacher said that his father had worked on a farm. Though this was entirely accurate, part of the family actually owned the farm and the Observer suggested that he had concealed his middle-class origins. Meacher sued for libel. Sadly for him, he lost the case, and the jury placed him firmly in the squirearchy. (Ten years on, at the countryside march, when he became the first minister to join a demonstration against his own government, Meacher was nicely turned out in tweeds and a Barbour jacket.)

It is impossible to imagine any such argument now. In the intervening period, the working class had lost its prestige. Electorally it no longer holds the balance of power, industrially it is broke and aesthetically it has become the most lampooned social group in Britain. All the advantages of being working class inside the Labour Party have been worn away, leaving the Deputy Prime Minister perched uneasily on that thin line between cherished relic and embarrassing fossil.

Prescott's use of language illustrates the point. Every article on him mentions his bad syntax. Bevin had bad syntax, too, and surprising pronunciation (once he accused a Tory minister of mouthing "clitch after clitch"). But his solecisms provoked admiration, not derision, because they reminded people where he had come from. In contrast, Prescott's tangled sentences condemn him; and his rough-house style is seen to be dangerously out of sync with the preferred voice of new Labour. Wasn't it the great spinner Peter Mandelson who poured cold water on the idea that Labour should have a quota system to ensure more working-class representatives in parliament? "Northern, horny-handed, dirty-overalled", was how he described them.

That's why Prescott is confused by his class. But there's still hope. The day after he officially joined the middle class, a newspaper doctored a picture to make it look as if he were drinking champagne at a party, not beer. He was on to them like a shot and extracted an apology. More of that, please, John.

The writer, a producer at Blast! Films, is working on a short series about Labour Party culture