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Another police force has sent a file on alleged Tory election fraud to prosecutors, it was revealed today.

The Crown Prosecution Service confirmed that it had received a file from West Midlands Police - the 13th police to do so in recent weeks.

The largest electoral fraud investigation in British political history began a year ago after the Mirror revealed that more than two dozen Conservative MPs allegedly failed to declare all their spending on the 2015 general election campaign.

A decision on whether to charge the MPs will be made within weeks.

A Mirror investigation revealed last March that battle buses packed with Tory activists were sent to help Tory candidates in more than two dozen other marginal constituencies in the last days of the election.

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But despite appearing to campaign for individual local candidates, thousands of pounds of costs on bus hire, hotel bills and food were registered as part of the Conservative Party’s national campaign.

This ensured that local candidates did not break strict local election spending limits.

Dozens of detectives working for 19 separate police forces have been investigating for nearly a year whether the costs should have been declared locally.

Two weeks ago, the Electoral Commission upheld our findings and ruled that some of the costs of the battle buses should have been declared by the candidates.

Failing to register all election expenses is a criminal offence under the Representation of the People Act and the accused MPs and their agents could face up to a year in jail and an unlimited fine if convicted.

Several of the accused MPs said last year: “My election expense return for the 2015 election was completed and returned by my election agent in accordance with the law.

“It included all items authorised by my election agent for use in my campaign. I signed the necessary declaration on that basis.

“I am aware that CCHQ campaigned across the UK for the return of a Conservative Government, including in seats such as mine.

“Such campaigning would be part of the national return not within our local return.

A Conservative spokesman said earlier this month: “We are cooperating with the ongoing investigations.”