The Conservatives are expanding their use of professional telephone marketing in their bid to win key constitutencies at the next election.

According to the Times, the party is focussing its campaigning efforts on 400,000 voters in vital marginal seats on which the election result could hinge.

"The whole point of this exercise is on a uniform national swing we would need a 10.5 per cent for the Tories to form a government. That's a mountain," one party source told the paper.

"Actually, what we need to do is work not for a uniform swing but specific swings in specific seats. We know where we need the votes."