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.
Boris Johnson has made an appeal to "kippers" thinking of voting for Ukip - saying "it doesn't seem to me there's a lot between us".
The Mayor of London said Douglas Carswell, a Conservative defector who became Ukip's first MP appeared to be a "doppelganger" with whom he agreed on most subjects.
He told BBC One's Andrew Marr Show: "What do you want? You want a low-tax economy - what Douglas stands for - you want global free trade, you want a libertarian approach on the economy [...] socially progressive? Go for the Conservatives."
"We're the only party, by the way, that will deliver what the Ukip people want: a referendum on Europe."
He said the Conservatives should campaign to leave Europe should reforms of Britain's relationship with the EU not take place as hoped by the party, and called for a "points-based" system of immigration.
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.