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.
Labour's Chuka Umunna has called on Ukip to address "racists" in its ranks after allegedly receiving abusive tweets from supporters of the party.
Asked about the messages on Sky News, the shadow business secretary said: "Of course there are racists in Ukip - that is unfortunate and that is something they need to address.
"They say terrible things about women, they say terrible things about different parts of our society."
Umunna said the party understood that immigration was a "concern" for many Brits - and a key campaign issue for Ukip - but insisted he would not say "I agree with Nigel" [Farage] on leaving the European Union and thus preventing freedom of movement into Britain.
"Europe is our biggest trading partner. Europe provides jobs. Our links with Europe provide jobs in this country and [Ukip] want to walk off with their bat and do nothing," he said.
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.