The SNP have called on the Conservative party chairman to step down while police investigate electoral fraud allegations.

SNP MP Pete Wishart said it was unacceptable for Lord Feldman, a close friend and ally of David Cameron, to remain in his post.

The Conservatives face a series of police investigations into claims they breached strict election spending rules by failing to record accommodation and transport costs for activists and officials campaigning in marginal seats.

Read more: SNP politician asks police to investigate Tories over electoral law-breaking allegations

As pressure increased yesterday a Tory MP also asked the elections watchdog to investigate Nicola Sturgeon's use of a helicopter during last year's General Election.

Charles Walker also reported Labour and Liberal Democrat to the Electoral Commission for their use of so-called battle buses.

The SNP insisted that a visit by a party leader was not the same as bussing groups of activists into target seats.

Mr Wishart said that the scale of the investigations into the Tories was "simply breathtaking".

He said that while the police investigations were ongoing "it cannot be business as usual for the Conservative Party".

The Tories had so far casually dismissed the allegations as ‘accounting errors’ or honest mistakes, he said.

"It is now unacceptable that Andrew Feldman remains in place when so many investigations are being conducted.

Read more: Tory MP asks elections watchdog to investigate Nicola Sturgeon's helicopter

"He was the chair of the Conservative Party when all these allegations were said to have taken place and there are questions about what he knew about the alleged disparity between candidate expenditure and the national campaign expenditure for which he was responsible.

"I have also asked the Metropolitan Police to investigate whether the alleged breaches of the representation of the people act was co-ordinated at a central office level.

"It would therefore be in the interests of everyone concerned that Andrew Feldman stands aside until all of this concluded."

In his letter, Mr Walker urged the Electoral Commission to "clarify its guidance" on campaign buses.

He also suggested that a failure to examine the Labour, Liberal Democrat and SNP campaigns could give the impression of political bias.

A commission spokesman said it would respond in due course, but said that the organisation's role was to regulate national party spending and that allegations "surrounding candidate spending returns are a matter for the police".

An SNP spokesman said: “Party leader tours are not the same as bussing in activists into marginal seats. We have correctly registered our party leader transport costs as such with the Electoral Commission."

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Official figures show the party spent more than £35000 hiring the helicopter to fly Ms Sturgeon around Scotland in the run-up to the election last May.