Welsh nationalists Plaid Cymru have been fined £29,000 by the elections watchdog after failing to report dozens of donations to the party.
The Electoral Commission said the party omitted 36 cash donations worth almost half a million pounds from its disclosures during a period of two years.
A spokesman for the watchdog said: "The commission's investigations pointed to a lack of effective internal processes, which led to a failure to understand the reporting requirements."
The fine is one of the larger ones dished out by the watchdog but not the largest in history.
Vote Leave, the official campaign for Brexit, was fined £61,000 for breaking electoral law in 2018 while Britain First was fined £44,000 over "multiple breaches of the law" last year.
The Welsh party, which has four MPs in Westminster, submitted nine inaccurate reports omitting a total of 36 cash donations worth more than £497,000. It belatedly reported the donations in May 2018.
The commission's director of regulation Louise Edwards 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.
"It is vital that voters can see full and accurate financial records that show where a political party's money comes from.
"Plaid Cymru'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.
"Voters can have confidence that, where parties fail to comply with the rules and there is no reasonable explanation, we will take action."
The commission said Plaid Cymru must pay a fine of £29,268 by March 2.
A Plaid Cymru spokeswoman said: “The matters highlighted by the Commission are 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.
"All public funds have been fully reported in the Party’s Annual Report.”
