The Conservative Party has been fined £70,000 ($85,922) for misreporting election expenses in 2014 and 2015, the Electoral Commission announced on Thursday (16 March).
The watchdog found that there were "significant failures" by the party to report accurately on how much it spent on campaigning at three by-elections (Newark, Rochester and Strood and Clacton) in 2014 and at the 2015 general election.
The Tories' general election spending return was missing payments worth at least £104,765, with payments worth up to £118,124 either not reported to the Commission or incorrectly reported by the party.
"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," said Sir John Holmes, the chair of the Electoral Commission.
"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."
The Commission also found that the Conservatives did not include the required invoices or receipts for 81 payments to the value of £52,924. Elsewhere, the party failed to maintain records explaining the amounts it invoiced to candidates in three 2014 by-elections, for work on their campaigns.
The probe was launched on 15 February 2016 after an investigation by Channel 4 News' Michael Crick, who looked into how parliamentary candidates registered the expenses of the Conservatives' "battle bus".
A dozen police forces across the UK have sent files to the Crown Prosecution Service (CPS) and two MPs, including South Thanet's Craig Mackinlay, have been questioned by the police.
Nick Timothy, one of Theresa May's top aides, has also been implicated in the expenses investigation by Channel 4 amid claims he was involved in Mackinlay's successful campaign to beat former Ukip leader Nigel Farage.
The Commission has also reported then-Conservative treasurer Simon Day to the Metropolitan Police over an alleged breach of election rules.