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.
Harriet Harman insists there has not been a "wobble" in the Labour party over flagging poll ratings.
Labour's Deputy Leader told The Andrew Marr Show: "There is no problem within the Labour ranks."
"We are not going to have a wobble or a leadership change," Harman said.
Asked whether Ed Miliband could be replaced as leader before the 2015 General Election, Harman replied: "No, absolutely not."
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.