DAVID Cameron's attempt to hang on to the marginal Midlands seat of Corby in today's crunch Westminster by-election has been branded a "shambles" after the MP heading the Tory campaign was secretly filmed apparently supporting a rival candidate.

Chris Heaton-Harris incurred the ridicule of Labour and the wrath of Conservative HQ after a Greenpeace activist recorded him revealing that he had suggested his friend James Delingpole should stand on an anti-wind farm ticket in the Northamptonshire constituency, which has been branded "Little Scotland" because of the 10,000 Scots who live in the former steel town.

Labour is looking to overturn a 1951 Tory majority in what would be the first Conservative-held seat to switch to it since 1997.

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Ed Miliband, who has made four visits to Corby, has ordered all 40 Scottish Labour MPs to be there today to ensure his party's support is maximised in the by-election that was triggered by the resignation of sitting Tory MP Louise Mensch.

Last night, the Opposition sought to deepen Mr Cameron's embarrassment by calling on him to withdraw the whip from Mr Heaton-Harris.

Michael Dugher, Labour's vice-chairman, said the Conservative leader's failure to discipline his colleague, who represents Daventry, for a "serious betrayal of his party" stood in stark contrast to the decision to suspend the whip from Mid-Bedfordshire MP Nadine Dorries after she flew to Australia to appear in the TV reality show I'm A Celebrity Get Me Out Of Here without seeking permission.

Last night, James MacKenzie, a former press officer for the Scottish Green Party, told The Herald he had referred the matter to Northants Police, questioning whether or not the 1983 Representation of the People Act had been broken and asking, for example, whether any donations from Conservatives had made their way to Mr Delingpole.

Theresa May, the Home Secretary, said Mr Heaton-Harris had been "guilty of some silly bragging".

Mr Delingpole announced his intention to stand in Corby but withdrew from the race two weeks ago after John Hayes, the Conservative Energy Minister, made clear the development of onshore wind farms would be reined in.