The UK Independence Party says it now has more than 36,000 members, putting it around 8,000 behind the Liberal Democrats.
The party, led by Nigel Farage, said it was on course to overtake Nick Clegg's junior coalition party before next year's general election.
Mr Clegg has appealed to pro-European Tories and Labour to counter Ukip's eurosceptic arguments ahead of this year's European Elections on 22nd May. Writing in The Guardian, Mr Clegg labelled Ukip as part of the anti-Brussels establishment.
He said: "Farage is every bit the professional politician he enthusiastically reviles. He and I were elected to the European Parliament on the same day in 1999. I left after five years. The Ukip leader is still there."
Nigel Farage has continued to defend a controversial £1.5m advertising campaign which has been branded racist by opponents.
Opponents have labelled the eurosceptic party's new billboard campaign 'racist', a charge Nigel Farage claims is a 'classic trick'.