The Electoral Commission has told MPs that it is satisfied that a £435,000 donation to the DUP was permissible under UK law.
Some £425,000 of the money from the Constitutional Research Council (CRC) was spent on pro-Brexit advertising throughout the UK.
The CRC is thought to be a group of pro-union business people chaired by Richard Cook.
Mr Cook is a former vice chairman of the Scottish Conservatives.
The commission's chief executive, Claire Bassett, and head of regulation, Louise Edwards, gave evidence to the Commons' Digital, Culture, Media and Sport Committee on Tuesday as part of the committee's inquiry into disinformation and fake news.
Ms Edwards told MPs that during the referendum period when the DUP received the controversial donation from the Scottish-based Constitutional Research Council the commission had received quarterly reports from the party about the donations they had received.
Under the rules which applied to donations to Northern Ireland parties at that time, the commission is not allowed to publish the identities of donors.
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
However, Ms Edwards clarified that "if we discover a donation in one of those reports is impermissible then the restrictions are lifted and we can talk about that donation".
"We cannot talk about donations to the DUP from that period and the reason for that is because having verified those reports the donors on them were permissible," she said.
Questioned by SNP MP Brendan O'Hara about whether the Electoral Commission had done everything it could to check the money which went to the DUP was not of foreign origin and was permissible under UK law, Ms Bassett replied "we were satisfied that the donors were permissible".
Mr O'Hara questioned the Electoral Commission about allegations made in a recent BBC NI Spotlight programme regarding whether there was a common plan between the DUP and the referendum campaign group, Vote Leave.
Ms Edwards said the Commission had taken the view there was insufficient evidence to warrant an investigation into the matter.
Who is Richard Cook?
- A prominent figure in Scottish Conservative circles, he stood as a parliamentary candidate in several elections - most recently in the Westminster seat of East Renfrewshire in 2010
- Mr Cook blogged in 2010 that he was a director of the think tank Think Scotland
- His company Cook Consulting (UK) Ltd signed a major agreement with Pakistan in 2012 to deliver environmental projects in Karachi worth nearly $1bn (£640m)
- Little is known about the pro-Brexit Constitutional Research Council, of which he is chairman