In the month since the European parliament elections, a growing number of voices on the left of British politics have warned that Labour is failing to grasp the significance of Ukip. After warnings from a shadow minister and the leader of Unison, the Fabian Society identified six Labour-held seats that Ukip – now the most working-class party in Britain – effectively "won" at the elections last month. In Rotherham, Rother Valley, Dudley North, Plymouth Moor View and Penistone and Stocksbridge, the speed of Ukip's advance, coupled with evidence of a broader decline in blue-collar support for Labour, led the Fabians to talk of a "considerable vulnerability to Ukip".

The 2014 election results were certainly a wake-up call for those who refused to believe that Ukip could inflict mass damage in Labour territory. The party that some progressives cheered on as a tool for "dividing the right" took over 41% in Rotherham and North East Lincolnshire, over 35% in Doncaster, Hartlepool, Middlesbrough, Scarborough, Redcar and Cleveland and Kingston-upon-Hull, and over 25% in places like Carlisle, Calderdale and Stockport. Such results have even led some inside Ukip to talk of a "2020 strategy" for establishing their party as the only long-term credible alternative in Labour heartlands. "This seat is not for 2015," explained one senior activist to us while flicking through a list of Ukip's targets. "It's for 2020 when the incumbent Labour government is on its knees."

While Labour took too long to acknowledge Ukip's potential, it is now failing to understand its appeal. Most think they can halt Ukip by talking about the cost of living, protecting public services like the NHS, defending the economic interests of blue-collar Britons and talking in vague and abstract ways about devolving power or rebuilding institutions. Last month Labour sought to frame a vote for Ukip as one for a Thatcherite party that would raise taxes, force voters to pay to use the NHS and see their local GP. Such a response is not surprising; it is rooted in the old Marxist belief that support for nationalist parties is driven by economic insecurity, and encouraged by capitalists who would prefer ethnic over class conflict. But this is not a long-term strategy and nor is it supported by actual evidence.

First, by the time Ukip arrives in Doncaster for its conference in September its policy review will most likely have neutralised these lines of attack by modifying its stance on issues such as the NHS. Moreover, targeting the healthcare policies of a party that spends all its time talking about migration and Europe just appears odd. Second, this economically focused narrative does not hold up when we look at the Labour voters who actually left Labour for Ukip last month, and who are thinking of doing so again in 2015.

Data from the latest British Election Study shows us that the voters who left Labour for Ukip were older, poorly educated white pensioners who hold a very different outlook from Labour MPs or thinktankers. But as the chart below shows, they were not driven to Ukip by the NHS or cost of living. Foremost, it is immigration and European integration that dominate the minds of voters who switched from Labour to Ukip at the European elections (yellow), and especially those who intend to now stay with Ukip (black). These Labour deserters are also more likely to think that important social changes such as giving equal opportunities to ethnic minorities and same-sex couples have gone too far. In short, those who have left or who are thinking of leaving Labour for Ukip don't like modern Britain for social and cultural reasons – not economic.

Chart

The good news for Labour is that only a small number of its 2010 voters actually switched to Ukip in 2014 (about one in 10). But focusing only on this misses a fundamental longer term trend. There are now lots of angry, old white voters in Britain and Ed Miliband's Labour is no longer their preferred destination. Between 2005 and 2013 Labour support among white working-class pensioners slumped from 45% to just 26%. In the same period, Ukip support among this group surged almost tenfold, from 3% to 28%.

Simply talking about economic issues will not win these angry old white voters back, in the same way that social democrats across Europe talking only about the economic benefits of migration or European integration have not neutralised the radical right. While Labour has now finally recognised the challenge that Ukip represents, it must now devote serious effort to making sense of the underlying causes.