Pearson said his party had targeted six Tories and Labour's David Drew, in Stroud, who Ukip believe would ask questions both inside and outside the Commons chamber on Europe and defy the party whip "if necessary".

At a press conference in central London today, Pearson named the Conservative candidates Ukip would not run against as Philip Davies (Shipley), Douglas Carswell (Clacton), Janice Small (Batley and Spen), Alex Story (Wakefield), and Philip Hollobone (Kettering).

It will also not run against independent Bob Spink in Castle Point.

The strategy is part of Ukip's ongoing pursuit of a referendum on Europe.

"We are not just standing aside," said Pearson. "What we going to do is rather different; we are going to actively campaign for all these candidates."

In Stroud, where Drew is campaigning for a fourth term, the Ukip candidate had already registered but was "asking voters to back the Labour candidate if possible", said Pearson.

Pearson conceded that none of the Conservatives or Drew had actually asked for Ukip's support. "I think it is possible there are some other seats here before the end of the campaign where our candidates – where we have them – say: 'If you possibly can, you vote for mostly the Conservative candidate or if not hold your nose and vote for me.' That will draw the votes ... We think this is different. I don't think this [has been] done before."

www Ukip Uncovered