The Conservatives have been fined a record £70,000 after an investigation into gaps in the party's campaign spending accounts.
The Electoral Commission said it had found "significant failures" by the party in accurate reporting of how much it spent on campaigning for three by-elections in 2014 and at the 2015 general election.
Investigators found that the party has listed spending which should have counted towards individual candidates' budget - which, it concluded, gave them a "realistic prospect" of "financial advantage" over opponents.
Video report by ITV News Political Correspondent Carl Dinnen
One matter has also been referred to the Metropolitan Police for further investigation.
The prime minister told ITV News Political Editor Robert Peston that the party had "complied fully" with the Electoral Commission.
The investigation found:
£104,765 of payments were missing from the 2015 general election spending return
Separately, up to £118,124 was either not reported or was inaccurately reported
81 payments to the value of £52,924 were missing the required invoices or receipts
At least some of the reported £38,996 Battlebus expenditure should have counted towards candidates' budgets, rather than counted as party spending
£63,486.83 of Battlebus expenditure was not included in the report due to "human error"
Records explaining the amounts invoiced to candidates were not maintained
The Commission also accused the party of "unreasonable uncooperative conduct", which it said had delayed the release of information vital to the investigation.
It is the third fine imposed on one of the biggest political parties for inaccurate reporting of campaign expenses.
Labour and the Liberal Democrats were both fined £20,000 last year for undeclared spending in the 2015 general election.
Theresa May told ITV News that the Conservative Party would pay the fine, adding that there were some issues the party raised with the commission through their investigations.
Claire Bassett, chief executive of the Commission, has now called for the regulator to be given power to dish out greater penalties.
She told ITV News that fines in the "hundreds of thousands, rather than the tens of thousands" would be more appropriate - and revealed investigators had wanted to fine the Conservatives more, had they been able to.
Chairman of the Commission, Sir John Holmes, echoed her sentiments - saying more powers were needed to punish political parties who break the rules, to stop them seeing fines as just a "cost of doing business".
The rules established by Parliament for political parties and their finances are there to ensure transparency and accountability.
It comes after the Crown Prosecution Service revealed it had received files from at least 12 police forces relating to general election expenses in 2015.
The investigation was opened in February last year after concerns were initially raised by Channel 4 News that the Tories' spending reports may be incomplete.
Officials found that party treasurer Simon Day, who left the post in April last year, had committed three breaches of the Political Parties, Elections and Referendums Act 2000 (PPERA).
The matter referred to police revolves around Mr Day's obligation to sign a declaration that, to "the best of his knowledge and belief", the return was complete and accurate.
"The investigation established that the party’s general election return was neither complete nor correct, and the evidence gathered during the course of the investigation has given the Commission reason to suspect that an offence may have been committed," the Commission said.
"It will be a matter for the police as to what steps they take following the Commission’s referral."
The party has until April 13 to pay the fine.
A Conservative Party spokesman said it would pay the fines imposed without argument.
However, he called for a review of the Electoral Commission's requirements, saying they could be "clarified" to avoid similar errors in future.
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.
He also disputed that any Battlebus spending should have been attributed to candidates in specific constituencies, saying there was "no reason not to declare it" in the national return.
“MPs in constituencies visited by the battlebus would have no reason to consider whether it should be included in their local return – they were directed that the bus would be visiting as part of CCHQ’s national spending," he added.
“The ongoing investigations relate to national spending by CCHQ and the national Party will continue to co-operate with the police and other authorities so that the matter can be resolved as soon as possible.”