Ukip is sucking up voters Tories can ill afford to lose

It is little wonder that Tory MPs are worried about Ukip. Nigel Farage's party is more popular than ever before, and its rise could enable Labour to claim more than a dozen Tory scalps in 2015.

Ukip won just over 3 per cent of the vote in 2010, outperforming the previous best Eurosceptic performance, the 2.7 per cent won by the now defunct Referendum Party in 1997.

As recently as January Ukip was still at 3 per cent on average in the polls. But, as Tory fortunes have waned during this year, so Ukip has profited.

Now the party is averaging 6 per cent. Earlier this month it secured a record 14 per cent of the vote in the Corby by-election.

Not all of Ukip's new converts voted Tory last time – but many did. On average the polls suggest 7 per cent of 2010 Tories have switched to Ukip, compared with 4 per cent of Liberal Democrats and just 1 per cent of Labour voters.

If those figures were to be reflected in the 2015 ballot boxes, it would generate a 1 per cent swing from Conservative to Labour – enough to have changed the outcome in 13 seats last time around.

David Cameron cannot possibly afford such a penalty if he is to win a majority in 2015.

John Curtice is Professor of Politics, Strathclyde University