UK Independence Party leader Nigel Farage is expected to announce today whether he will take up Nick Clegg's challenge of a public head-to-head debate over whether Britain should remain in the European Union.
The Deputy Prime Minister threw down the gauntlet during a live radio broadcast in which he called for a showdown which Liberal Democrats said would pit the leaders of the "party of In" and the "party of Out" against one another.
There was no immediate acceptance of the challenge from Mr Farage, who indicated that he would like to see Prime Minister David Cameron and Labour leader Ed Miliband invited to join them in a four-way clash.
But Ukip promised a "full" response from its leader when he appears on LBC 97.3 radio this morning to take listeners' questions.
Mr Clegg used his regular weekly phone-in on the radio station to declare: "I will challenge Nigel Farage to a public, open debate about whether we should be in or out of the European Union.
"He is the leader of the party of Out, I am the leader of the party of In.
"And I think it's now time we have a proper public debate so that the public can listen to the two sides of the argument and judge for themselves."
The move came as Lib Dems brace for what the party's president predicts will be "the fight of our lives" in elections to the European Parliament on May 22.
Tim Farron has warned that the party's "very presence in the European Parliament could be at stake" as it contends with poor opinion poll ratings which consistently put the junior coalition party behind Eurosceptic Ukip.
A Ukip spokesman said: "Mr Farage would like to thank Mr Clegg for his kind invitation to a debate on the great issue of Britain's membership of the European Union.
"Perhaps he could also let us know whether he has invited David Cameron and Ed Miliband too in order that the British people can see all their main political leaders argue their positions.
"If this challenge means that Mr Clegg is going to restore his backing for an In/Out referendum, which he gave before the last election but then withdrew afterwards, then it could be a significant moment in British politics."
Mr Clegg has already stated that Mr Farage should be allowed to take part in televised debates between party leaders during the 2015 general election campaign.
The Prime Minister has made clear, however, that he does not believe Ukip should be represented if agreement is reached to repeat 2010's ground-breaking TV showdowns.
Following Mr Clegg's challenge, a Lib Dem spokesman said the terms and arrangements for any debate with Mr Farage "would have to be agreed by both sides".
A spokesman for Mr Clegg dismissed the Ukip calls for a wider debate on the basis that neither the Conservatives nor Labour had a "clear position" on the Europe issue.
"This is a simple idea: a debate between Nick Clegg and Nigel Farage," he said.
"While we completely disagree, both parties have a very clear position: Nick Clegg leads the party of In and Nigel Farage leads the party of Out.
"There's really not very much point debating the other two - neither of them have a clear position.
"The Conservatives can't make up their minds and many of them want to drag us towards the exit, while Labour don't have the courage of their convictions on this, they won't lift a finger to keep Britain in the EU.
"So let's keep things simple and have the debate. In versus out; Lib Dems versus Ukip; Nick Clegg versus Nigel Farage."