Media coverage of the London elections focused, inevitably, on the victory of Mayor Boris. But even with a close-fought race pushing up turnout to a new high, the slow rise of the BNP continued as they gained a foothold in the London Assembly. This should not merely be a cause for concern, but for us to reflect on how our own party can respond.

The BNP polled 5.3 per cent across London and averaged 13.9 per cent in the 642 council wards it contested around the country. It now has 55 councillors and poses a serious threat across several regions in next year's European elections, not to mention the mayoral election taking place in Stoke-on-Trent next year.

There are some who still think that the BNP is a flash in the pan that will disappear as quickly as it emerged. But we are facing a shift in British politics. Traditional voting patterns are fragmenting as voters shop around for a party that best articulates their concerns and even prejudices. The emergence of the BNP is just one consequence of this change.

Labour's support among its working-class electoral base has been shrinking for many years, and this goes well beyond the recent decline in the government's fortunes. In many areas, Labour's support among the working-class C2DE demographics was at a lower level in the 2005 general election victory than in the crushing defeat of 1983. Since then, support among these groups has further disintegrated.

Some of these disappearing voters switched to other parties. In local elections this was often the Liberal Democrats, but far greater numbers simply stayed at home.

For the Labour leadership, this long-term shift caused only moderate concern. It is a truism that general elections are not won or lost in the Labour heartlands but in the swing marginals, where a few votes can turn success into defeat. It is towards these voters that the major parties have calibrated their language, tactics and policy.

The New Labour project relied on the assumption that its traditional support, although declining, had nowhere else to go. But this is now changing, and the BNP has emerged as one beneficiary. The party received more votes last month than Labour in seats such as Dagenham and Rainham in east London and the new Morley and Outwood constituency in West Yorkshire.

But we cannot view the BNP in isolation. In other areas, such as south Yorkshire and South Wales, it has become represented by the rise of local independents. Who would have thought that Labour could have lost the former heartlands of Merthyr Tydfil and Blaenau Gwent? In Stoke-on-Trent, a city where ten years ago Labour held all 60 seats, it was this year able to win only four wards. In Barnsley, where the BNP polled 21 per cent, the Barnsley Independent Group holds a third of the seats on the council.

This trend reflects a more fundamental shift than midterm blues. An increasing number of traditional Labour voters believe that the party no longer reflects their interests. This is in no small measure a result of new Labour's triangulation tactic - a deliberate shift to what the political class thinks is the "centre ground". It is also a symptom of a failure to prioritise grass-roots activism at the local level, instead flirting with the "virtual party" and delivering messages through centralised marketing. The danger is not only that we ignore the reasons for the strength of the BNP, but that in so doing we reinforce the very conditions that have created it.

Despite the generally benign economic climate of recent years, many of the people now turning their back on Labour have not shared in this economic prosperity. Swaths of these voters not only feel ignored but have been persuaded that the BNP articulates their interests.

Race is obviously the vehicle through which the BNP galvanises support but the party also articulates the frustration of many voters and seeks to provide them with a new sense of belonging. As politicians remove class as a social, economic and political category, the BNP seeks to insert race.

It is no coincidence that the BNP is doing best in those communities, often overwhelmingly white, where there has been the greatest economic change, such as the former coalfields and car manufacturing areas. For too long a basic formula has underscored much New Labour thinking - a counterbalancing of so-called aspirational, Middle-England swing voters with our traditional supporters. Its adherents have remained tone deaf to both the aspirations and insecurities of those who fall outside this tight political calculus.

Ministers' rhetoric of "aspiration" fails to address the real aspirations of voters across huge tracts of the political landscape, where even decent housing or good jobs are in too short supply. So our language, policies and tactics all fail to hit the mark.

All this represents a fundamental shift in British politics, and the real danger is that we are heading the way of many continental countries where large segments of the working class have broken with their traditional centre-left parties and moved to the right, often the far right.

The Labour Party always had a mission of emancipatory economic and social change but to many it feels like we have lost our traditional purpose or even identity. The economic downturn, the credit crunch, the housing collapse and rising living costs will only increase material insecurities over the next few years.

There is still time to move in a different direction but without a radical and immediate change Britain, and in particular Labour, could fall victim to the same political rupture that has already shaken much of modern Europe.

Jon Cruddas is MP for Dagenham

Nick Lowles is editor of Searchlight