An exclusive Channel 4 report on alleged electoral fraud by the Conservative Party has prompted eight separate police forces to launch investigations into the matter.

The allegations centre on last year’s general election campaign and whether costs, in the tens of thousands, to transport Tory activists to marginal seats in a “battle bus” and set them up in accommodation should have been declared on individual candidate’s expenses rather than falling under the national campaign.

According to Channel 4, the story is the result of a three-month long investigation by its news team. The channel’s political correspondent, Michael Crick, first reported the matter in a blog post in late January. A second report, claiming the Tories had overspent on three by-elections, followed in early February. As more Tory candidates became embroiled in the affair Channel 4 news anchor John Snow broadcast a report with 20 empty chairs, showing ministers who had turned down requests for interview from the Bafta-winning programme, on April 21 ahead of local, mayoral and national elections on “Super Thursday” last week.