Dear Labour Left,

We all know how the fall of the Berlin Wall changed politics. The collapse of the Soviet empire was swiftly followed by the collapse of traditional communism. True, the world’s most populous country is still a one-party communist state. But far from levelling incomes – either up or down – China is now more unequal than any western democracy and it is actively pursuing new goals. Instead of aiming to bury America, it is now busy buying America. The ideological war between capitalism and state socialism is over. Capitalism, though not necessarily its democratic variant, has prevailed.

Here in Britain, most of us who support Labour have either adjusted to this new world, or know we have to. But there is another big adjustment that many of us on the Left have yet to make. It cannot be linked to a single moment in history; no dramatic photographs have recorded it: perhaps the nearest is the iconic picture of Margaret Thatcher standing alone in a derelict Middlesbrough industrial estate in the late Eighties. 'It' is the collapse of social class as the main determinant of the way people vote.

The two tables below (spreadsheet version here) provide the numbers – or, rather, the best numbers that I can find: the first in absolute numbers, the second in terms of percentages. They are derived from opinion polls from the last eleven general elections. Polls are not always perfect, so allowances should be made for some margin of error. In addition, the figures for “abstentions” must be treated with some care. They show the total electorate at each election minus the number of votes cast. But we know the electoral register is inaccurate – around three million adults are not on the register; on the other hand, the register does include people who have died since it was drawn up (and therefore are not really 'abstentions' but appear so in the statistics), those registered at more than one address, and those who have moved house (who are technically abstentions but cannot be blamed so readily for not voting).

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However, making every possible allowance, we find that a remarkable shift has taken place. In 1970 more than four-fifths of Labour’s vote was working class, while less than one-fifth was middle-class. That is, 10m members of the C2, D and E social classes vote Labour, compared with 2.2m of the A, B and C1 social classes. (Broadly speaking people are rated as ABC1 if the head of their household has a white-collar, professional or non-manual job, and C2DE if the head of their household has a blue-collar, manual job or lives on state benefits.)

This year, for the first time, more middle-class than working-class people voted Labour.

Two profoundly reshaping things have happened.

The first is that the social structure of Britain has changed: forty years ago, working-class adults outnumbered middle-class adults by almost two-to-one. Today, middle-class Britain outnumbers working-class Britain by four-to-three.

The second change is that people are far less inclined to vote along class lines than they used to. In 1970, 56% of working-class voters backed Labour, compared with just 22% of middle-class voters – a 'class gap' of 34 points. This year, that gap has been reduced to just six points: 27% of middle-class voters backed Labour, compared with 33% of working-class voters.

On the Conservative side, the story is much the same: a 'class gap' of 33 points in 1970 has turned into a gap this year of just seven points.

If we look at abstentions, we find that throughout the Seventies and Eighties, middle-class electors were more likely to vote Tory than to abstain, while working-class voters were more likely to vote Labour than to abstain. Neither thing is now true. In this year’s election, middle-class abstainers outnumbered middle-class Tories by 7.3m to 6.5m, while among working-class electors, the 8.6m abstainers outnumbered the 4.2m Labour voters by two-to-one.

One response to these figures is to say that Labour needs to re-energise its traditional base. If it returns to pre-New-Labour policies and campaigning methods (so the argument runs), it can revive its electoral fortunes.

I believe this would be disastrous. Yes, Labour does need to do more to win back the C2DE votes it has lost since 1997. The mistake is to think that the way to achieve this is through a pre-Blair, class-based style of politics.

The reason is two-fold.

First, Britain no longer has the economic and social structure it used to have and will never have again. In 1950, not only were a large majority of Britons working class, but those in work were largely men who did tough and often tedious manual jobs in our factories and mines. They rented their homes and owned few possessions – the car and television age had not yet arrived. Labour’s post-war reforms in health, housing, pensions and national insurance spoke to their aspirations; its values reflected theirs.

In Barnsley, as much as 89% of the electorate turned out in that year’s general election. They returned Frank Collindgridge to Westminster. Why such a high turnout? It was not because Barnsley was a knife-edge marginal. Mr Collindridge secured 42,008 votes; his majority was 31,209. Rather it was because the act of voting, and voting Labour, was of a piece with being a miner. Mr Collindridge himself had been one. He had started working down the pits at the age of thirteen, 44 years earlier.

Compare that with what happened in Barnsley Central this May. It is still a safe Labour seat; its MP, Eric Illsley, also has a mining background – though as an official of the National Union of Mineworkers, rather than as a miner. He is a graduate of the University of Leeds, rather than the University of hard knocks. Turnout in 1950 was 89%; in 2010 it was 56%. Mr Illsley won 17,487 votes, not 42,008; his majority was 11,093, not 31,209.

Similar comparisons can be made in other cities and industrial areas. Turnout is sharply down – more so than in more well-to-do Britain – and among those who do vote, the proportion voting Labour is also sharply down. It is tempting to believe that if only we could revive the spirit of the Attlee years, Labour would be unstoppable.

That spirit, however, was the product of a different era. Britain has changed in fundamental ways: the patterns of work, the nature of consumption, the character of our culture, the respective roles of men and women – all these would be unrecognisable to the 1950 generation. Labour’s male, union-dominated, collectivist from of politics made sense then; it makes no sense today. This is not a new observation: from Hugh Gaitskell to Tony Blair, attempts have been made to reposition Labour to attract the voters of modern Britain – and with notable, if temporary, success in Mr Blair’s case.

But old habits die hard: the instinct remains that Labour’s main task is to revive support among “our people” – a proposition that implicitly assumes that the old social divisions still persists, albeit in slightly different forms.

My contention is that this is simply not true. The roots and forms of social solidarity that dominated the Labour Party for the first fifty years of its life, and which lingered in progressively weakened forms for the next thirty, have simply gone. True, there are pockets, sometimes quite large pockets, of people whose economic circumstance prompts many of them, when they vote at all, to vote Labour: people living in social housing, for example, or public sector manual workers. But theirs is an instrumental choice about what would be best for them and their family; social solidarity has little or nothing to do with it.

Because, and this is the second change, whether you qualify as working class or middle class according to the traditional A to E classification, the fact is that, for better or worse, the same shared consumer-led culture dominates British life. In a way we are a culturally more cohesive society than we have ever been.

OK, leave out the top ten per cent and the bottom twenty percent, while granting that these are important sections of our population. For the overwhelming majority of us, our shared experiences are not of pain, struggle or oppression, or of the collective institutions that helped people to survive and fight back – trade unions, working-men’s clubs, the Co-op and, sometimes, churches. Today’s shared experiences and irritations are of credit cards, Tesco, mobile phones, MoT tests, Sky, mortgage-lenders, commuting, the Internet and the X-Factor. In ways that go far beyond anything David Cameron has said, the vast majority of us are indeed in this together.

Again, that is not a new proposition. But its political consequences have still not been fully absorbed, especially by the Left. That is one reason why so many, and so many more since he left Downing Street, reject Tony Blair and his classless, inclusive approach to British politics. Yet when he said he wanted the Labour Party to be “the political wing of the British people”, his ambition was exactly right.

It will be fatal if this is ignored. For example, Neal Lawson and John Harris have just called for a “New Socialism” in this week’s New Statesman, accusing me wrongly of wanting to take Labour to the Right. But they seem to live in their own heuristic world, oblivious to the way people actually live, work, think and feel; making the terrible mistake of simply ignoring the huge changes in economy and society that have taken place in the past few decades. Since when has wanting to strip millionaires of their Freedom Pass and winter fuel allowance been a right-wing proposition? It would be a tragedy if, in reaction to Iraq, Blair’s approach to civil liberties and his disdain for many Labour traditions, the Left also rejects the big thing he got right. The past is another country, and it’s here and now in which we have to win elections, not there and then.

This still leaves plenty of room for those of us on the Left to debate the nature of progressive politics in a post-ideological, post-class-war era. My views and conclusions may differ greatly from yours. But it would help if at least we argued on the basis of a broadly agreed contemporary reality, rather than rival nostalgic myths: a reality that can be seen in the figures set out in the two tables above.

Peter Kellner is President of YouGov and one of Britain's leading psephologists.