Plaid Cymru has been fined £29,000 for failing to report cash it received from taxpayers’ funds worth nearly £500,000.
The Electoral Commission said over a two-year period Plaid had omitted 36 separate sums from quarterly reports.
Plaid had failed to declare cash from the House of Commons authorities, and some cash from the Electoral Commission.
A spokesman for the party said it corrected the oversight when it was notified and “is now in compliance”.
The fine must be paid within a fortnight.
Opposition parties that win MPs at Westminster are entitled to so-called “short money”, which they can use to help them with their costs.
The Political Parties, Elections and Referendums Act 2000 states that “any payment out of public funds received by a registered party shall be regarded as a donation received by the party from a permissible donor”.
According to the Electoral Commission, the party submitted nine inaccurate reports over a two-year period, omitting what it described as 36 cash donations worth more than £497,000.
The commission said Plaid then reported the donations in May 2018. Investigations “pointed to a lack of effective internal processes, which led to a failure to understand the reporting requirements”, it said.
Louise Edwards, director of regulation at the Electoral Commission, said: “The total number and value of donations omitted from Plaid Cymru’s quarterly reports is significant and reveals a substantial degree of non-compliance.
“Plaid Cymru is a well-established party and it should be able to meet its reporting obligations.”
She added that the party’s “sustained failure over a two-year period to understand and meet its legal duties led to a disappointing lack of transparency into the party’s finances”.
A Plaid Cymru spokesman said the matters highlighted were “historic in nature and do not involve any private donations”.
“As soon as Plaid Cymru were notified, the Party immediately corrected this oversight and is now in full compliance,” he said.
“All public funds have been fully reported in the party’s Annual Report.”