Cameron faces Ukip by-election bloodbath: Party set to win first Commons seat after shock poll reveals Farage's staggering 44-point lead over the Tories

Most worrying of all for the Prime Minister is the reaction of other potential Tory MP defectors to the likely outcome of the by-election.

If Mr Carswell lost or won narrowly, other like-minded Conservatives toying with going over to Ukip would probably conclude it was far too risky.

But if he wins with a record-breaking swing, as our poll suggests he could, they may decide it is more risky to fight the next election as a Tory candidate than as a Ukip one.

And that could change the landscape of British politics.

Pollster Damian Lyons Lowe breaks the good news in the Mail on Sunday.