The Conservatives have blamed Labour for the prospect of a historically strong performance by the British National party in Thursday’s elections, as new research suggests support for the far right is strongest in Old Labour heartlands.

The Tory intervention marks a shift in the traditional bipartisan approach to fighting the BNP. All three main party leaders are urging people to shun the far-right fringe party amid fears a low turnout could combine with protests against MPs’ expenses to give the BNP at least one seat in the European parliament.