The DUP has confirmed it received a Brexit donation of about £435,000 from a group of pro-union business people led by a Conservative party member.
The money from the Constitutional Research Council was spent on pro-Brexit advertising throughout the UK, said DUP MP Sir Jeffrey Donaldson.
The CRC is chaired by Richard Cook, a former vice chairman of the Scottish Conservative and Unionist Party.
Mr Donaldson said about £425,000 was spent on the Brexit campaign.
The DUP said they were given more money than they spent and the remainder of the donation, around £9,000 was transferred to the party's funds with the agreement of the Constitutional Research Council.
Speaking on the BBC's Good Morning Ulster programme, Mr Donaldson stressed it was "not a Northern Ireland referendum, but a UK-wide referendum".
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
A total of more than £32m was spent on the campaign - with the Leave side funded by donations totalling £16.4m, outgunning the Remain side's £15.1m.
During the referendum campaign, the DUP took out a four-page "Vote To Leave EU" advertisement in the Metro newspaper - which is available in London and other cities but not in Northern Ireland.
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
Earlier this month, DUP leader Arlene Foster declined to reveal the identity of the donor or donors but said the money was properly accounted for "under the rules as they currently stand".
While political donations in Northern Ireland are kept confidential for fear of identifying donors, other parties had called on the DUP to clarify who funded its "lavish" pro-Brexit advertising campaign.
Mr Donaldson said his party had "reached millions" by spending the money on media outside of Northern Ireland.
He denied the DUP had broken the spirit of electoral law.
"Absolutely not, the DUP registered as a national campaign organisation for the referendum," he said.
"The law is very clear, the political parties and referendums act allows any political organisation to register nationally to participate in a campaign.
"We recognised that this referendum was going to be won or lost on a national basis. That's why we spent money advertising in the Metro free sheet because it would reach far more people.
"This way we reached millions of people and the feedback we got back from our campaign was very, very positive."