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.
Any Conservative-Ukip pact ahead of next year's General Election would look to voters like a "stitch-up", Boris Johnson has said.
Appearing to rule out the move, the Mayor of London said: "The whole point of this great movement against Westminster politics is that everybody thinks it's a stitch-up.
"How is it going to help if we all go round trying to stitch things up [...] I don't see how that works, no."
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.