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David Cameron has apologised after he said facing the shadow chancellor in Parliament was "like having someone with Tourette's sitting opposite you".

"I was speaking off the cuff and if I offended anyone, I am very sorry," he told the ���˿���'s Andrew Marr.

The prime minister initially made his comments about Ed Balls's heckling

Nicky Clark, whose daughter has Tourette syndrome, said his comments showed "a lack of understanding".

Tourette syndrome is a neurological disorder characterised by tics - involuntary, rapid, sudden movements that occur repeatedly.

It can be particularly debilitating because the vocal tics can include the uncontrollable use of obscene language - known as coprolalia - and repetition of phrases the person hears others use - called echolalia.

In the Sunday Telegraph interview, the prime minister said of Mr Balls: "He just annoys me. But I'm very bad, in the House of Commons, at not getting distracted, and the endless, ceaseless banter, it's like having someone with Tourette's permanently sitting opposite you."

Speaking on the Andrew Marr Show, Mr Cameron said Prime Minister's Questions could be "challenging".

"It is a lesson for me that in the Commons I have to try and tune out the noise and just concentrate on trying to answer the question," he added.

Mrs Clark, who is from Shropshire, told ���˿��� 5 live's Breakfast show that "comments like this compound the myths and the stereotypes surrounding people with Tourette's in Britain".

She added: "If we're supposed to look to him as some kind of lead, is this the best we can expect and disabled people expect?

"This comment comes in the middle of a piece about a fairer Britain. How fair is Britain going to be for disabled people if this is the lead we get from Downing Street?

"This shows an utter disregard for the condition and a lack of understanding from our prime minister."