The Conservative Party has been fined £70,000 by the Electoral Commission for failing to accurately report its election expenses.
The independent elections watchdog said the party had made "numerous failures" in reporting its expenses for the 2015 General Election and three by-elections in 2014.
Commission chairman Sir John Holmes said the Tories' failure to follow the rules "undermined voters' confidence in our democratic processes" and said there was a risk political parties were seeing such fines as "a cost of doing business".
The fine follows the news that a dozen police forces have sent files to the Crown Prosecution Service as part of a probe into the Conservatives' 2015 election expenses.
At least three Tory MPs have been quizzed by police investigating whether election finance laws were broken in the 2015 contest.
The parallel investigation by the Electoral Commission found that the Tories' 2015 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, a portion of which was recorded as national party spending when it should have been recorded as candidate spending in individual constituencies.
In addition, the Tories did not include invoices or receipts for 81 payments worth £52,924 and failed to maintain records explaining the amounts it invoiced to candidates in three 2014 by-elections for work on their campaigns, meaning the accuracy of the sums could not be verified.
Sir John 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.
"The rules established by Parliament for political parties and their finances are there to ensure transparency and accountability.
"Where the rules are not followed, it undermines voters' confidence in our democratic processes, which is why political parties need to take their responsibilities under the legislation seriously."
He went on: "This is the third investigation we have recently concluded where the largest political parties have failed to report up to six-figure sums following major elections, and have been fined as a result.
"There is a risk that some political parties might come to view the payment of these fines as a cost of doing business; the Commission therefore needs to be able to impose sanctions that are proportionate to the levels of spending now routinely handled by parties and campaigners."
The Commission said there was a "realistic prospect" that the party gained a "financial advantage" over opponents in the 2015 general election.
It follows Channel 4 News allegations that the Tories had incorrectly recorded spending on a "battle bus" tour which took activists to campaign in key marginal seats.
The allegations centred on whether the tour should have been recorded as counting towards individual candidates' spending limits, rather than as part of the larger national spending return.
Among the seats investigated by the Commission was South Thanet, where Tory Craig Mackinlay narrowly beat former Ukip leader Nigel Farage.
In the report on its investigation, the Commission said: "The advantage obtained by party by its actions; the inclusion in the party return of what in the Commission's view should have been reported as candidate spending meant that there was a realistic prospect that this enabled its candidates to gain a financial advantage over opponents.
"In this respect the Commission noted that the Battlebus2015 campaign visited target constituencies and that South Thanet was also a key priority for the party."
The Conservatives said they had complied fully with the Commission's investigation and would pay the fine that has been imposed.
"Political parties of all colours have made reporting mistakes from time to time," a party spokesman said.
"The Labour Party and Liberal Democrats both failed to declare sums of money which constituted a larger proportion of their national expenditure in the 2015 general election. Both have been fined by the Electoral Commission, and the Liberal Democrats are also under police investigation.
"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."