The Liberal Democrats have been fined £18,000 for breaching campaign spending rules during last year's Brexit referendum, the elections watchdog announced.
The majority of the fine was for failing to provide acceptable invoices or receipts for 80 payments worth more than £80,000, the Electoral Commission said.
The Britain Stronger in Europe official campaign to Remain in the EU, which has become Open Britain, was also fined £1,250 for failing to deliver a complete and accurate spending return.
Bob Posner, Electoral Commission director of political finance and regulation, said: "The reporting requirements for parties and campaigners at referendums and elections are clear, that's why it is disappointing that the Liberal Democrats didn't follow them correctly.
"The major political parties must ensure their internal governance is sufficiently invested in and resourced so they can be sure of meeting their legal obligations.
"Where the rules are not followed, transparency is lost, which is not in the public interest or as Parliament intended."
The Lib Dems, who campaigned to remain in the EU, were fined £17,000 for failing to provide acceptable invoices or receipts - in some cases none were provided at all, and in others some were "inadequate", such as credit card statements, or incomplete.
The second fine of £1,000 was issued because some payments were reported in aggregate rather than as individual payments, the commission said.
A Lib Dem spokesman said: "The Liberal Democrats always endeavour to provide complete reports of national campaign expenses in good time and according to all of the applicable rules.
"The mistakes that have occurred in this case are a result of human error and we are taking the necessary steps to ensure these mistakes are not repeated in future."
Open Britain has now paid fines of £1,000, because some payments in its previous guise as Britain Stronger in Europe were reported as aggregate rather than individual payments, and £250, in relation to three invoices that were not provided.
The commission also fined the Immigrants Political Party £2,500 for late delivery of its 2016 statement of accounts and quarterly donations and loans report.
Traditional Unionist Voice was fined £1,850 for late delivery of pre-poll reports for the 2017 general election, 2016 statement of accounts, and campaign spending return for the 2017 Northern Ireland Assembly election.
The Labour Campaign for Human Rights was fined £1,350 for late reporting of donations by a members' association.