David Cameron told ITV News that Lynton Crosby has not lobbied him over government policy.
The Prime Minister told UK Editor Lucy Manning; "He does not advise on government policies and he hasn't lobbied me on any government policies."
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Charities warn that sitting down too long can lead to health risks such as type 2 diabetes and cardiovascular disease.
The Labour leader will use a speech to set out the differences between his party and the Conservatives with particular emphasis on the NHS.
Kate will join Prince William on a tour of the Stephen Lawrence Centre before visiting a charity working with disadvantaged young people.