Mr Clarke said the party had no positive policies and was 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 attack 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 — said it did not condone “unpalatable views”. A spokesman insisted dredging up old activities on social media amounted to a “political lynch mob”. Mr Clarke said: “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 if he agreed with David Cameron’s 2006 assessment that the party was packed with “fruitcakes and closet racists,” Mr Clarke said: “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.”