A dozen police forces have sent files to prosecutors as part of a probe into the Conservatives' 2015 election expenses as Tory MPs urged the party's leadership to take action on the issue.

The Conservative Party was this morning fined £70,000 by the Electoral Commission for breaking election expenses rules with its "battle bus".

At least three Tory MPs have been quizzed by police investigating whether election finance laws were broken in the 2015 contest.

Tory MP Anne-Marie Trevelyan said she hoped the "very difficult" situation would be resolved "very quickly".

She said: "I think the whole situation is very difficult and I hope very much that it will be resolved this week.

"I understand from colleagues that we are into that stage and the party chairman will be doing all he can to make sure that the right outcome and that those MPs who did nothing wrong, they followed the appropriate electoral law, have this really difficult pressure taken off them and that there is a full and satisfactory conclusion to the whole process."

The Berwick-upon-Tweed MP told Channel 4: "The issue of the bus and that being national spending was very clearly made at the time.

"We need to make sure that all parties get that through the system and actually we have a fair and satisfactory conclusion which means that those MPs who did nothing wrong, whose election expenses are absolutely as the rest of ours were, are not tarnished."

Morecambe and Lunesdale MP David Morris is one of three MPs known to have been interviewed by police.

"Lancashire Police interviewed me and they saw fit not to take it any further," he said.

He said he did not want a visit from the battle bus campaign at the centre of many of the claims about election spending breaches.

The Conservatives have insisted that the busloads of activists sent to key seats formed part of the national campaign spend rather than falling within the lower constituency limits.

Mr Morris told BBC's Newsnight: "We were all given an email, every one of us had the same email from Mark Clarke at the time who was running the battle bus project, saying it was a national spend."

He added: "I honestly believe not one MP is guilty of anything."

He said his case was straightforward: "We did not want the battle bus, and that was said from day one.

"We got the battle bus, we were instructed to have the battle bus, which is the same for everybody else, which is what the parties do."

The disclosure follows reports that Craig Mackinlay, who fought off a challenge from former Ukip leader Nigel Farage to hold Thanet South for the Tories, had been questioned for six hours last week by Kent Police.

Colchester's Will Quince also revealed that he had been interviewed by police and told he faced no further action.

Mr Quince voluntarily attended an interview under caution with Essex Police last January.

Essex is not one of the forces that has submitted files to the CPS.

But a CPS spokesman said it had received files from: Avon & Somerset, Cumbria, Derbyshire, Devon & Cornwall, Gloucestershire, Greater Manchester, Lincolnshire, the Metropolitan Police, Northamptonshire, Nottinghamshire and West Yorkshire police.

Once it has received a file, the CPS will decide whether to charge anyone.

The files were "all under consideration", a spokesman said.

A 12th force - Staffordshire - said it had also sent a file to the CPS.