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.
The Ukip leader told BBC1's Sunday Politics:
The price would be a full, free and fair referendum on our continued membership of the European Union, the opportunity to get our country back, and for that to happen quickly.
It means the eurosceptic party would agree to a confidence and supply arrangement, where it backs or abstains on budget and legislative programme votes.
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.