The Labour Party has been fined £12,500 by the Electoral Commission over failings to deliver accurate and on time declarations about donations.
The watchdog said Labour had failed to deliver an "accurate quarterly donations report" and "accurate weekly pre-poll reports" from April-June 2017.
It said it was the "highest fine we have issued" for that kind of offence.
Labour said it had been a "genuine administrative error" and it adhered to all rules regarding donations.
Political parties are obliged to submit a report on donations received every three months, as well as a weekly account in the six weeks prior to a general election.
But the Electoral Commission said Labour had failed to do this on both counts in a period prior to the snap election in 2017.
It has fined the party £12,000 for a "failure to deliver" an "accurate quarterly donations report" and £500 for a "failure to deliver accurate weekly pre-poll reports" for the period covering April, May and June 2017.
The snap election was on Thursday 8 June 2017.
BBC political correspondent Chris Mason said he understood the party failed to submit at least one donation in the appropriate report - submitting it three months later than it should have - and there were duplications in its submission as well.
Neither the Labour Party nor the Electoral Commission have set out how big the relevant donations were, or from whom.
Louise Edwards, director of regulation at the Electoral Commission, said: "The sanction we have imposed on the Labour Party for an inaccurate donations report is the highest fine we have issued for an offence of this kind.
"As a well funded political party it should be able to meet its legal requirements."
She added: "The reporting requirements are clear, so it is always disappointing when parties and campaigners fail to comply.
"The law clearly states when and how financial data is required to be submitted to the commission.
"We will continue to enforce these requirements on all parties and campaigners to ensure voters have the information they need."
The Labour Party said: "This was a genuine administrative error. Labour adheres to all Electoral Commission rules on the publication of donations to the party."
Labour has already paid the fine.