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The Conservative Party has been fined a record £70,000 for breaking election expenses rules.

The party insists its failure to report six figure sums it spent on trying to win three by-elections and the general election was an "administrative error".

The Electoral Commission said there was a "realistic prospect" the money had given the party an advantage.

The Metropolitan Police is now looking at the evidence to see if the reporting omissions were deliberate.

The party broke spending rules by moving campaigners and staff from its national headquarters to boost local party efforts and not properly declaring their hotel bills and expenses.

The investigation, which followed Channel 4 News' investigation, found:

The Conservative Party's 2015 UK Parliamentary general election spending return was missing payments worth at least £104,765

Separately, payments worth up to £118,124 were either not reported to the commission or were incorrectly reported by the party

The party did not include the required invoices or receipts for 81 payments to the value of £52,924

The party failed to maintain records explaining the amounts it invoiced to candidates in three 2014 by-elections, for work on their campaigns

The successful Conservative campaign in South Thanet to see off a challenge by then UKIP leader Nigel Farage at the 2015 general election was among those criticised in the commission's report.

The Electoral Commission found the Conservative Party also failed to correctly report all expenditure on a national battlebus campaign, which helped David Cameron win a majority at the general election.

It has referred a possible criminal offence - of whether Simon Day, the Conservative Party's registered treasurer until April 2016, "knowingly or recklessly made a false declaration" - to the Metropolitan Police.

Labour and the Lib Dems have previously been fined for breaking election spending rules prompting the Electoral Commission to warn that "there is a risk that some political parties might come to view the payment of these fines as a cost of doing business".

Speaking at a charity event in London on Thursday evening, Mr Cameron said he had won the general election "fairly and squarely", and that he was happy with a statement made earlier by Conservative head office.

The Conservatives said "there needs to be a review of how the Electoral Commission's processes and requirements could be clarified or improved".

Analysis - by Political Editor Laura Kuenssberg

Seventy thousand pounds is a lot of money, but in the context of a political campaign where millions of pounds are spent, it's not exactly going to break the Tories' bank. But the political cost of what might happen next is much higher.

Thirteen police forces are now looking at whether the mistakes made might constitute criminal offences. If that was to happen, there could be by-elections in seats around the country, that could seriously affect the PM's unhealthily slim majority in Parliament.

And the whiff of financial wrong-doing is an odour no political party wants. But how likely is that actually to happen?

Senior Tory sources tell me they think it's unlikely the mistakes, and there were plenty of them, will reach the hurdle for the prosecution. The CPS has to believe there is a good chance of a successful conviction, and while this is speculation, senior Tories don't believe in most of the cases that's likely.

Read Laura's blog in full

Q&A: Conservative election expenses row

The Commission's chief executive Claire Bassett told BBC Radio 5 live the investigation had taken much longer than necessary because of "some difficulties" in getting information from the Conservative Party.

She added that having had to get a court order to get information was "very disappointing".

Electoral Commission chairman Sir John Holmes said: "Our investigation uncovered numerous failures by a large, well-resourced and experienced party to ensure that accurate records of spending were maintained and that all of the party's spending was reported correctly."

He added that failure to follow the rules "undermines voters' confidence in our democratic processes".

But senior Conservative MP Oliver Letwin said that spending record "mistakes" were probably down to "human error".

Media playback is unsupported on your device Media caption CEO of Electoral Commission tells Today they don't know if Tory expenses misreporting was deliberate

Media playback is unsupported on your device Media caption Conservative MP Sir Oliver Letwin tells Today expenses misreporting probably down to human error

"I don't think you should conclude from this that there is some great conspiracy," he told BBC Radio 4 Today's programme.

The Electoral Commission's investigation covered the national party rather than spending by individual candidates, which local police forces have been looking into.

Twelve police forces have asked the Crown Prosecution Service to consider charges over election expenses.

BBC Political Editor Laura Kuenssberg said that if prosecutions go ahead "we could be looking at by-elections".

In response to the Electoral Commission report, a Conservative spokesman said it had "complied fully... and will pay the fines".

"This investigation and these fines relate to national spending by CCHQ, and the Conservative Party's national spending return for the 2015 general election.

"As we have consistently said, the local agents of Conservative candidates correctly declared all local spending in the 2015 general election.

"CCHQ accepted in March 2016 that it had made an administrative error by not declaring a small amount constituting 0.6% of our national spending in the 2015 election campaign.

"Political parties of all colours have made reporting mistakes from time to time... this is the first time the Conservative Party has been fined for a reporting error.

"We regret that and will continue to keep our internal processes under review to ensure this does not happen again.

"Given the range of technical errors made by a number of political parties and campaign groups, there also needs to be a review of how the Electoral Commission's processes and requirements could be clarified or improved."

UKIP's chairman Paul Oakden said: "As UKIP has always said, the laws are in place in order to ensure that big and wealthy political parties cannot buy British politics."