Some potential voters for the UK Independence Party were denounced as racists by a senior Tory minister today as relations between the two parties plumbed new depths of acrimony.

Amid alarm in Conservative high command over the likely performance of the resurgent anti-European Union party in Thursday’s local elections, the Tory high command has authorised a drive to highlight the far-Right background of some of Ukip’s 1,730 candidates.

But Kenneth Clarke, the minister without portfolio, widened the Tory attack to accuse some people who vote Ukip of being driven by racial prejudice.

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His comments brought a furious riposte from the Ukip leader, Nigel Farage, who accused him of “contempt for the ordinary men and women of this country”.

He told The Independent: “Ken Clarke looks and sounds like nothing more than a bloated remnant of the ancien regime, plucking crumbs from his velvet waistcoat and disdaining the people beyond the gates.”

Mr Clarke referred to David Cameron’s depiction six years ago of Ukip as being full of “fruitcakes, loonies and closet racists”.

He told Sky News: “I’ve met people who satisfy both those descriptions in Ukip. Indeed some of the people who assure me that they are going to vote Ukip, I would put into that category. And I rather suspect that they have never voted for me.”

Mr Clarke described some Ukip candidates as “clowns” and “indignant, angry people” and added: “Fringe-right parties do tend to collect a number of waifs and strays.”

Ukip has accused the Conservatives of “dirty tricks” by trawling through council candidates’ social media sites to uncover embarrassing comments about any of its candidates.

It said it does not condone extreme views, or links to organisations such as the British National Party or English Defence League, but protested that the Tories were acting like a “political lynch mob”.

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There were fears in Conservative circles tonight that Mr Clarke could have gone too far by labelling some voters as racist and senior Tory sources refused to echo his comments. But they were unrepentant over the focus on Ukip candidates’ past views, insisting that such close examination was essential when people were standing for public office.

Patrick McLoughlin, the Transport Secretary, said he did not know anything about a “smear campaign”, but added: “Why should Ukip candidates get away without any scrutiny when other parties have scrutiny as far as their candidates are concerned?”

Ukip’s surge in the national opinion polls has dismayed the three biggest parties, each of which has seen its supporters attracted to Mr Farage’s colourful attacks on the political establishment.

But the Tories have been hit hardest by Ukip and fear a strong showing by the party in Thursday’s elections could cost them dozens of seats.

In the final few days of campaigning, Conservative activists have been advised by Central Office to warn wavering Tories that defection to Ukip would “let Labour in through the back door”.

Although there has been a relative spell of calm in Conservative ranks in recent weeks, a disastrous performance at the ballot box could revive internal criticism of David Cameron’s leadership. They are braced for about 300 losses, but more than 500 could set alarm bells ringing again among Tory MPs.

Critics have warned that a challenge to Mr Cameron cannot be ruled out if the party, which was beaten by Ukip at the recent Eastleigh parliamentary by-election, continues to haemorrhage support at the ballot box.

The Liberal Democrat leader Nick Clegg urged voters to examine Ukip’s policies carefully.

"I think the more you look at Ukip in terms of what they stand for, people would be a bit less attracted when they realise that Ukip's stance in favour of further cuts, meaning there will be cuts to your local schools and hospitals,” he said.

Hilary Benn, the shadow communities secretary, said some of its candidates appeared "pretty unappealing".

He said: “As Ukip have certainly risen in the polls they are being subjected to a lot more scrutiny, and quite right too.

“If you are going to stand for elected office then you should be expected to be scrutinised for the views that you hold, including the views you expressed in the past, so I don’t know what they are complaining for.”

On the offensive: What top Tories say about UKIP

Ken Clarke: "Fringe right parties do tend to collect a number of waifs and strays... It is very tempting to vote for a collection of clowns or indignant, angry people, who promise that somehow they will allow us to take your revenge on people who caused it.” Yesterday

Patrick McLoughlin, Transport Secretary: "Why should Ukip candidates get away without any scrutiny when other parties have scrutiny as far as their candidates are concerned?” Yesterday

Grant Shapps, Conservative Chairman: "Vote Ukip and you will let in a Labour MP and put Ed Miliband into Downing Street.” January 2013

Michael Fabricant, Conservative Vice-Chairman: "The Conservative Party might well win the 2015 General Election on our own. But a pact with Ukip on clear terms could deliver 20 extra seats. I have had no contacts with UKIP. But we cannot ignore the maths.” (November 2012)

David Cameron: "Ukip is sort of a bunch of ... fruitcakes and loonies and closet racists mostly. I don’t think I’m saying anything that hasn’t been said before.” (April 2006)