A pro-Brexit group which gave £435,000 to the DUP during the EU referendum campaign broke electoral law by failing to report the donation.
The Constitutional Research Council was fined £6,000 for failing to notify the Electoral Commission.
Following an investigation, the CRC declared the 2016 donation and the commission found the source of the money was permissible.
The latest details are contained in new correspondence from the Electoral Commission released by the Good Law Project.
'No reasonable excuse'
The correspondence confirms for the first time the CRC was fined £6,000 for failing to notify the commission of political contributions it made as well as gifts it received.
In the letter, the commission stated the CRC "had no reasonable excuse for these failings".
The CRC is chaired by Richard Cook, a former vice chairman of the Scottish Conservative and Unionist Party.
Mr Cook was contacted by BBC News NI but declined to comment.
The Good Law Project, founded by Jolyon Maugham QC, is a campaign group which works to uphold the rule of law. One of its central aims is to try and stop Brexit.
The organisation has been seeking a judicial review of the Electoral Commission over its decision not to investigate the handling of the DUP's biggest ever donation.
The commission's letter from 19 October 2018 was responding to the Good Law Project's threat of a potential judicial review.
Of the £435,000 donated to the DUP, £425,000 was spent on the Brexit campaign and the remainder was transferred to the party's funds.
The DUP did report the donation and has consistently insisted it complied with electoral law at all times
A party spokesperson said: "We have nothing to add to our previous comments on this matter."
How the £425,000 was spent
- £282,000 on advertising in Metro newspaper in support of Brexit
- £99,616 on promotional material
- £32,750 with Canadian IT and consultancy firm
- £10,823 spent in Northern Ireland
The names of those who donated the money to the CRC have never been released.
Donor laws in Northern Ireland state that any donations made before July 2017 must remain secret.
However donor secrecy laws prevented the release of any further information. The Open Democracy website subsequently reported that the fine related to the CRC.
However the latest correspondence from the Electoral Commission confirms for the first time that the pro-Brexit group was fined because it did not initially report the donation it made to the DUP to the commission.
The fine also related to the fact the CRC failed to register any gifts of over £7,500 that it had received.
The Electoral Commission is prevented by law from commenting publicly on political donations in Northern Ireland made before July 2017.
However, for the purpose of responding to potential judicial review proceedings, it disclosed information to the Good Law Project in writing to defend claims it had not investigated the CRC donation.
In a statement to BBC News NI, the Electoral Commission said: "We are disappointed that the law continues to restrict us from providing the public with information on donations reported to us before July 2017.
"The prohibition does not apply to the Good Law Project, however we have asked them to use the information solely for the purpose for which it was provided."
The letter from the commission confirms that in July 2016 the DUP reported receiving two donations totalling £434,981 from the CRC:
- £99,988 on 9 June 2016
- £334,993 received on 21 June 2016
The correspondence from the commission states: "The information we received showed that the donations were from permissible sources. There was therefore no basis to consider investigating the DUP for failing to return an impermissible donation."
However, the letter also reveals that the CRC failed to notify the commission that it had made the donations to the DUP within the appropriate timeframe.
Under electoral law, the CRC should have notified the commission of its donations to the DUP by 8 July 2016 and reported the gifts it received by 8 August 2016, but failed to do so.
An investigation by the commission found that the CRC "had no reasonable excuse for these failings".
In the letter to the Good Law Project, the commission said: "The CRC told us it was not aware of its reporting obligations but that it now recognised that it should have notified the Commission and the error was inadvertent."
The commission fined the CRC £6,000, but said it also verified that it and its donors were permissible.