David Cameron branded the UK Independence Party "fruit cakes" and "closet racists" today.
The Tory leader attacked Ukip after it said it would use the Freedom of Information Act to try to force him to reveal backers who had secretly lent the Conservatives money.
He said it was just trying to make mischief, telling London radio station LBC radio: "Ukip is sort of a bunch of ... fruit cakes and loonies and closet racists mostly".
Mr Cameron's outburst will be a little uncomfortable for his chief spin doctor George Eustice, who once stood for Ukip in the European elections.
Ukip later demanded an apology from Mr Cameron.
Euro MP Nigel Farage said: "For Mr Cameron to resort to this combination of petty name-calling and disgraceful smears is hardly a statesmanlike approach for someone who hopes to become the next but one Prime Minister.
"We are a non-racist, non-sectarian party whose offence, in Mr Cameron's eyes, has been to attempt to force his party to disclose its sources of finance.
"We simply will not accept this type of smear being used simply to cover the Tories' blushes on a different subject, and he should apologise, not just to us, but to the 2.7 million voters who supported us in 2004."
He added: "Mr Cameron needs to learn that this sort of language in the 21st century is simply unacceptable. Fruit cakes and loonies we can live with - we have a sense of humour - but we draw the line at his unfounded accusations of racism."
Also in the LBC interview, Mr Cameron admitted the number of black Tory MPs was "pathetic". He pointed to just one black and one Asian Tory in the House of Commons and said: "I admit it's pathetic. In a party of 200 people we should be far more representative of the country that we want to govern."
He said he was determined to overhaul the way the party selected its candidates.
Mr Cameron said at the last election the Tories had more black and ethnic minority candidates than any other party. But he said he recognised it had to do more.Reuse content