Carswell, the eurosceptic Conservative MP for Clacton in Essex, today announced that he is joining UKIP.
The MP said he will resign his seat and force a by-election, which he will fight as the UKIP candidate. He made the announcement at a press conference in central London alongside his new party leader, Nigel Farage.
“I am standing here precisely because the Conservative leadership is not serious about change,” he said.
Clacton, a faded seaside resort on the Essex coast, is one of UKIP’s main target seats. Carswell currently has a 12,000-vote majority there over Labour.
Carswell said he had not informed David Cameron in advance of his announcement, insisting he wanted to “shake up UK politics”.
“The problem is that many of those at the top of the Conservative Party are simply not on our side,” he added. They aren’t serious about the change that Britain so desperately needs.”