Ukip has "fruitcakes, loonies, waifs and strays" in its ranks and among its supporters, Kenneth Clarke has said after a spate of stories questioning the credentials of the party's candidates in this week's local elections.

Ukip described the stories – which it claimed were part of a smear campaign by Conservative central office – as "morally reprehensible", but the transport secretary, Patrick McLoughlin, said he knew nothing about a smear campaign, arguing that all parties needed to be subject to scrutiny.

Ukip's leader, Nigel Farage, has admitted that his party has been overstretched in vetting its 1,700 candidates. During the local election campaign Farage has also jettisoned, seemingly on his whim, longstanding policies such as a flat rate of tax.

Speaking on Sky's Murnaghan programme on Sunday, Clarke, the minister without portfolio, abandoned the previous strategy of ignoring Ukip, accusing the party of creating "a quite ridiculous scare about the hordes of Bulgars and the hordes of Romanians who are coming here, which is total nonsense".

He said the government had to do more to prevent immigrants accessing some health benefits.

But he added: "The idea that hordes of benefit-claiming Bulgarians are heading to the United Kingdom is the sort of daft issue which Ukip, parties like Ukip, raise, and until we actually get to the stage where people discover that their neighbourhood isn't suddenly full of Romanians, obviously they will continue to exploit it."

Asked if some Ukip members were "fruitcakes and loonies", as David Cameron alleged some years ago, Clarke said he had met Ukip members who fitted that description.

He went on: "Indeed, some of the people who have assured me they are going to vote Ukip I would put in that category and I rather suspect they've never voted for me."

He said many Ukip supporters were angry about the failure of the political class. The lure of Ukip for voters, he said, "is these are very difficult times, the political class are regarded as having got us into a mess. It's very tempting to go for a collection of clowns or indignant, angry people who promise that somehow they'll allow you to take revenge against the people who caused it."

He was scathing about the quality of some Ukip candidates. "The fringe right parties do tend to collect a number of waifs and strays. Some of the ones they've sent to the European parliament, one of them got sent to prison, others had to send back a lot of money because they all believed what they were saying about the Brussels gravy train and rather unwisely tried to take advantage of it.

"But the trouble with Ukip, really, is it's just a protest party – it's against the political parties, the political classes; it's against foreigners, it's against immigrants, but it doesn't have any positive policies. They don't know what they're for."

Clarke warned of the dangers of electing people without political experience. "If you look at Italy … it shows the dangers if ordinary members of the public who are very angry about the political class, really, vote for people who are, in Italy's case, just comedians – a man called Beppe Grillo – and all sorts of men and women, some of them reputable, some of them not, who've no idea of government or what they wanted to do … you create a crisis, a bit of a mess. No doubt Ukip people are perfectly nice when they're having a drink but I wouldn't send most of them to the county council."

Clarke also showed a marked difference in tone from Tory rightwingers on spending by saying: "We're cutting spending more slowly than almost any of the other western European sovereign debt-infected countries. We're cutting our public spending slower than President Hollande's government in France and he was elected on an anti-austerity programme.

"We're coping and our head's above the water but we've got one or two, three years even, still to go. We've got to be allowed to do this job."

He said it might take between one and three years for the economy to recover.

Hilary Benn, the shadow communities secretary, claimed that Ukip's appeal was primarily among former Tory voters. He said: "It's quite clear that David Cameron has a lot more to worry about because all the evidence is that Ukip is taking a lot more votes from the Conservatives than they are from us or anybody else."

Paul Nuttall, Ukip's deputy leader, said: "All they can find is a problem with half a percent of our candidates." He added that any current or past BNP members found in Ukip ranks are thrown out. The party has a membership of 18,000, he said, and was adding 100 members a day.