The Liberal Democrats have been fined £18,000 for breaking spending rules in last year's EU referendum.
The Electoral Commission said the party had "failed to deliver a complete and accurate spending return".
Proper receipts and invoices were not provided for 80 payments worth more than £80,000, the watchdog said.
The Lib Dems said the breaches of the rules were down to "human error" and that the party was taking steps so they are not repeated.
The Electoral Commission is responsible for regulating election spending and political donations in the UK.
It has recently launched several investigations into spending in last year's referendum, which resulted in a vote to leave the European Union.
Announcing the Lib Dem fine, Bob Posner, the commission's director of political finance, said it was "disappointing" that the "clear" rules had been breached.
He added: "Where the rules are not followed, transparency is lost which is not in the public interest or as parliament intended."
The commission said the Lib Dems had not provided any invoices or receipts in some cases, while in others those provided were inadequate or incomplete.
The deadline for paying the fine is 3 January, 2018.
In response, 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."
Meanwhile the official Remain campaign, now known as Open Britain, has been fined £1,250 for wrongly reporting its spending.
Most of this is because of payments that were added together rather than being reported individually.