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.
The UK Independence Party's first elected MP has compared the Conservative party to HMV music stores, describing it as "defunct".
"It's a little bit like HMV music. Once ubiquitous when it came to buying music. The way the Tory party is retailing politics is like the way HMV retailed music - it is a defunct retail model," Douglas Carswell told the BBC's Andrew Marr show.
Carswell, who won the Clacton by-election with a huge majority last week, said he argued that "we needed to Spotify" the Tory Party, but found too much opposition to change.
He also played own suggestions Ukip could gain 25 MPs at the next election, saying: "Let's keep a sense of perspective....I don't like bravado talk and Ia'm not going to use it."
Carswell he was hopeful fellow Tory defector Mark Reckless would win the upcoming by-election in Rochester, saying a win would show that British politics is becoming "fundamentally different".
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.