ED Miliband has sought to up the ante in the row over David Cameron's relationship with his election guru Lynton Crosby by demanding a top-level inquiry into whether or not the Prime Minister has broken the ministerial code.

The code states that ministers "must ensure no conflict arises or could reasonably be perceived to arise between their public duties and their private interests, financial or otherwise".

Given that Mr Crosby's company has campaigned against minimum alcohol pricing and plain tobacco packaging in Australia and that the Coalition has, coincidentally, dropped its plans on both these policies, Labour has cried foul over what it says is a "devastating conflict of interest".

But the Tory high command insists Mr Crosby's company has a contract with the Conservative Party to advise on election strategy and has not lobbied any minister on policy.

During a fractious Prime Minister's Questions, Mr Miliband claimed Mr Crosby had influenced the UK Government's decision to shelve plain tobacco packaging in England and Wales; the SNP Government intends to go ahead with it in Scotland.

He accused Mr Cameron of having "caved in to big tobacco" and branded him the "Prime Minister for Benson and Hedge funds".

The PM then hit back at Mr Miliband over alleged union vote-rigging in Falkirk.

Asked if he had discussed plain tobacco packaging with Mr Crosby, whose clients include cigarette giant Philip Morris, Mr Cameron declared: "He has never lobbied me on anything."