Ken Clarke has launched a furious attack on Ukip, branding its politicians "clowns" and some supporters racist.
The Tory Cabinet minister said the party had no positive policies and were merely "against" foreigners and immigrants.
He also insisted it was perfectly acceptable for candidates in this week's local elections to be picked up on dubious comments they previously made on Twitter and Facebook. The swipe came after Ukip accused the Conservatives of running a "morally reprehensible" smear campaign against its would-be councillors.
The party - which is investigating a handful of its record 1,700 candidates over links to groups such as the BNP and alleged racist and homophobic comments - has said it does not condone "unpalatable views". But a spokesman complained that dredging up old activities on social media amounted to a "political lynch mob".
Mr Clarke told the Murnaghan show on Sky News: "They of course have not been able to vet their candidates. Fringe right parties do tend to collect a number of waifs and strays... Some of them are saying quite different things now they are in politics than their actual views."
Asked whether he agreed with David Cameron's 2006 assessment that the party was packed with "fruitcakes and closet racists", Mr Clarke replied: "I have met people who satisfy both those descriptions in Ukip. Indeed, some of the people who have assured me they are going to vote Ukip I would put in that category. I rather suspect they have never voted for me."
Mr Clarke said he was sure that "most of the Ukip people are perfectly nice when they are having a drink". But he insisted: "The trouble with Ukip really is it is just a protest party. It is against the political class, it is against foreigners, it is against immigrants. But it does not have any very positive policies. They do not know what they are for."
He continued: "The temptation to ordinary voters to Ukip is these are very difficult times. The political classes are regarded as having got us into a mess. 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."
However, Ukip MEP Paul Nuttall told the the BBC's Sunday Politics: "If the Conservatives have got people in central office doing these kinds of dirty tricks, going through the private lives of our candidates, going back to years on social media, it's sad, it's dirty, we're not going to respond."
Asked whether it was right to highlight any Ukip candidates with "neo-fascist" sympathies, he said: "We are the only political party in Britain which has it written into its constitution that if you have been a member of a far-right organisation you can never be a member of Ukip."