The Better Together campaign against Scottish independence and the British National Party (BNP) have been fined thousands of pounds for failing to submit financial information to the Electoral Commission on time.
The commission has fined Better Together 2012 Limited £2,000 for failing to deliver a complete campaign expenditure report for the Scottish independence referendum in 2014.
It has also fined the BNP £2,400 for failing to deliver its statement of accounts for 2014 by the due date.
The campaign expenditure report Better Together delivered did not include invoices or receipts to support approximately £57,000 (5%) of its expenditure, the commission said.
The law requires receipts or invoices for all items of expenditure in excess of £200.
The commission decided a financial sanction was appropriate as Better Together had "more significant resources and a higher profile than other campaigners".
Commission officials found "no evidence to suggest that the omissions were deliberate and took into account that the responsible person for the organisation made efforts to provide further supporting documentation after the deadline for the report had passed".
Better Together paid the fine in full on January 12.
Bob Posner, director of party and election finance and legal counsel at the Electoral Commission, said: "The responsibilities of a registered campaigner in a referendum do not end on polling day.
"Transparency around who spends what at referendums, based on complete and accurate financial reports, is an important part of ensuring that the public has confidence in the democratic process".
The BNP was granted a one-month extension to the July 7 2015 deadline for submitting its accounts because it was experiencing difficulty in finding an auditor, but the accounts were not delivered until October 16 2015.
The commission acknowledged the party experienced difficulty in finding a new auditor and that the party's treasurer had some health problems which exacerbated the situation, but concluded this and other issues could have been dealt with in a way that did not delay delivery of the accounts.
Commission officials took into account that the party had recently been sanctioned for other compliance failures and this is reflected in the level of the fine.
Any penalties that are imposed by the commission go into the Consolidated Fund managed by HM Treasury.