Nigel Farage has claimed Ed Miliband will be forced to back calls for an automatic EU referendum if Ukip outperform Labour in the upcoming European elections.
The Ukip leader told the BBC's Andrew Marr Show there was growing Labour backbench support for holding an in-out referendum, despite the Labour leadership clearly opposing it.
Mr Miliband told the same show that Labour would back a referendum if powers were transferred from Britain to the European Union, a situation he judged "unlikely but ... possible".
But he said debating whether we should exit the EU was not "a priority" among his plans to govern Britain.
Mr Farage, meanwhile, refused to be drawn on whether he would run as an MP at the next General Election after opting against a candidacy in the upcoming Newark by-election.
"I want Ukip to win the European Elections," he said. "We'll talk about the General Election afterwards."