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Eight police forces are investigating whether Conservative MPs filed election expenses illegally after the 2015 General Election, the BBC understands.

The allegations centre around failing to register the accommodation costs of party activists who were involved in the "battle bus" operation.

The party has blamed an "administrative error".

A Conservative spokesman said the party had already brought the error to the attention of the Electoral Commission.

The activists on the party's battle bus targeted marginal seats.

'Campaign activity'

The police probe will ascertain whether the expenses for the people using the bus should have been filed by the MPs' agents in their local expenses.

Failing to file correct expenses is a criminal offence.

The eight forces actively investigating are West Yorkshire Police, Derbyshire Police, Greater Manchester Police, Devon and Cornwall Police, Gloucestershire Police, Northamptonshire Police, Staffordshire Police, and Cheshire Police.

A further four forces are waiting for further information, the BBC understands.

Police can investigate such claims up to a year after the election expenses were filed, and West Yorkshire Police, Derbyshire Police, and Greater Manchester Police have started the process of getting extensions to the time limit.

A Conservative spokesperson said: "As is apparent from our national return, the party declared expenditure related to our CCHQ-organised Battlebus.

"However, due to administrative error it omitted to declare the accommodation costs of those using the vehicles. This is something we have already brought to the attention of the Electoral Commission in order to amend the return.

"The party always took the view that our national Battlebus, a highly-publicised campaign activity, was part of the national return - and we would have no reason not to declare it as such, given that the party was some millions below the national spending threshold.

"Other political parties ran similar vehicles which visited different parliamentary constituencies as part of their national campaigning."

Sanctioning

The Electoral Commission - which is an independent body that sets standards for running elections - is also investigating whether the Conservatives met their reporting obligations under the Political Parties Elections and Referendums Act during the general election.

The Commission has powers in relation to national campaign spending, although its sanctioning powers are limited to a civil penalty of up to £20,000.

Bob Posner, director of party and election finance at the Electoral Commission, said: "The police and the CPS [Crown Prosecution Service] both have the power to apply to the courts to extend the time limit on bringing criminal prosecutions for electoral offences to allow for full investigations to take place.

"We have requested that they consider doing this."