The Lib Dems have been fined £18,000 by the Electoral Commission for failing to provide invoices for campaign spending.
The party failed to produce invoices for more than £80,000 as a permitted participant at the EU Referendum, the commission found.
The total sum, which is almost the maximum individual fine of £20,000, actually comprises two fines.
The first, of £17,000, was for failing to provide acceptable invoices or receipts for the 80 payments. The commission said that in some cases no invoices or receipts were provided at all, and in others those that were provided were inadequate, such as credit card statements, or were incomplete.
The second fine, of £1,000, was issued because some payments were reported in aggregate rather than as individual payments.
Bob Posner, Electoral Commission director of political finance and regulation and legal counsel, 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.”
Open Britain, formerly the official remain campaign Britain Stronger in Europe, has been fined £1,250 also.
The organisation was fined £1,000 because some payments during the EU referendum campaign were reported in aggregate rather than as individual payments, and a further £250 in relation to three invoices that were not provided.
The Commission also published details of three further substantial fines imposed today:
Immigrants Political Party, fined £2,500 for late delivery of its 2016 statement of accounts, and quarterly donations and loans report;
Traditional Unionist Voice, fined £1,850 for late delivery of pre-poll reports for 2017 UKPGE, 2016 statement of accounts, and campaign expenditure return for the 2017 Northern Ireland Assembly Election;
Labour Campaign for Human Rights, fined £1,350 for late reporting of donations by a members’ association;
The deadline for payment is 3 January 2018.