David Cameron came under fire today after he claimed he knew nothing about a controversial Tory youth group which has trained some of the party's parliamentary candidates, despite its leaders describing the NHS as "the biggest waste of money in the UK" and suggesting that waterboarding of prisoners can be justified.

In an interview in Time Out magazine, Cameron denied all knowledge of the Young Britons' Foundation, which has been called "the Conservative madrasa". Conservative HQ promotes YBF-run training courses for young Tory activists.

Several members of Cameron's team, including shadow defence secretary Liam Fox, addressed a YBF rally last month and at least 11 prospective Tory candidates have been delegates or speakers at training conferences run by the YBF.

The group's leader, Donal Blaney, has taken Conservative activists on training courses in the US which include sessions shooting semi-automatic weapons and machine guns.

Asked if it was true that prospective candidates had been trained by the YBF, Cameron said: "No, I don't think so. I don't know anything about the Young Britons' Foundation." When told the YBF claims on its website that it trains Tory members at official party conferences, Cameron said: "I've no idea about this."

The party's link to the YBF was front page news last month and tonight Labour said Cameron's denial was "unbelievable" and called for him to "come clean" about his party's involvement with the YBF.

Labour MP Jon Cruddas said: "It is absolutely unbelievable that David Cameron doesn't know about an organisation that is training up candidates on behalf of the party – not least because its events have included contributions from party chairman Eric Pickles, shadow defence secretary Liam Fox, and other members of his frontbench team. Cameron should come clean on the role this shadowy organisation has at all levels of the party."

A spokesman for Cameron declined to answer questions about his depth of knowledge of the organisation, but confirmed it has "been involved with some Tory candidates". He added: "The YBF is independent of the party. We do not endorse their views."