Nick Clegg has dismissed Ukip's claim to be a party of political outsiders in an article describing it as an arm of the "longstanding Eurosceptic establishment".

Writing for the Guardian, the Lib Dem leader and deputy prime minister issued a rallying call to pro-Europeans and said that Nigel Farage's assertion that he was leading an army of "insurgents" was implausible in view of the strong current of Euroscepticism that existed in the Conservative party and in the media.

Instead, Clegg implied that it was the Lib Dems who were the insurgent underdogs, because they were confronting the "Eurosceptic establishment" directly in the European election campaign by fighting on an unashamedly pro-European platform.

He also insisted he had no regrets about his decision to hold two debates with the Ukip leader, despite polls showing that Farage was judged by viewers to be the clear winner of both, and he said he would be happy to have further debates before polling day on 22 May.

An ICM poll on Sunday suggested the Lib Dems were on course to achieve just 8% of the vote in the European elections, with the Conservatives on 22%, Ukip on 27% and Labour on 30%.

Many commentators expect Ukip to overtake Labour by polling day, with Farage's ability to portray it as an insurgent movement that is taking on the three main parties associated with the Westminster consensus a key factor in his party's success.

In his article, Clegg said this was untrue.

"Of all Nigel Farage's far-fetched claims – and there are many – the most outlandish is the idea that Ukip's call for EU exit is the insurgents' battle cry. European withdrawal is presented as a great revolutionary promise, held in stark contrast with the status quo offered by a homogenous political elite. What poppycock."

Clegg said Farage had been an MEP since 1999 and that, as a result, he was "every bit the professional politician he enthusiastically reviles".

He continued: "More importantly, there is nothing remotely new about his party's ambitions. Ukip are simply the fresh face of a longstanding Eurosceptic establishment, supported by many in the Conservative party and significant parts of our press."

Clegg claimed that the Eurosceptics had had a "free run" in the debate over Britain's relationship with Europe for too long.

"With such a bleak future [Britain leaving the EU] now entirely plausible, my party has decided to take on the Eurosceptic establishment directly," he said.

He also said that he would be happy to hold another debate with Farage. "I would meet the Ukip leader in front of the cameras every day between now and polling day if I could."

Patrick O'Flynn, Ukip's campaign director and director of communications, said: "It is laughable for Nick Clegg to claim the mantle of insurgency. Mr Clegg carries the title of deputy prime minister. All three of the parties with major representation in the House of Commons are in favour of maintaining Britain's membership of the European Union, as is most of the media."

Despite Clegg offering to hold another debate with Farage, Ukip believes that broadcasting rules governing election coverage would prevent such an event taking place now that the campaign is fully under way unless other party leaders were willing to take part.

On Tuesday, Farage will launch his European election campaign at a rally in Sheffield. Clegg will launch the Lib Dem campaign on Thursday in Colchester.