Ukip leader Nigel Farage has said his party would prop up a minority Conservative government if it agreed to a "full, free and fair referendum" on Britain's EU membership.
It follows a Survation poll for the Mail on Sunday which found that one in four Britons back Ukip.
The eurosceptic party won a by-election in Clacton with a giant 12,404-vote lead.
Conservative MP Zac Goldsmith has congratulated his former colleague and newly-elected Ukip MP Douglas Carswell on his win:
Douglas Carswell, who defected from the Conservative party and won back his seat as a Ukip candidate, has been introduced to the Commons.
As Ukip prepares to enter Parliament for the first time, leader Nigel Farage reflects on his party's byelection successes.
Ukip has its first elected MP after Douglas Carswell took almost 60% of the vote in the Clacton by-election.