A senior Democratic Unionist has said a donation used to fund a pro-Brexit campaign was raised legitimately.
The sum of £435,000 was given to Northern Ireland's largest party by the Constitutional Research Council (CRC), a little-known Great Britain-based group of pro-union business figures, ahead of the EU referendum last year.
Part of it was used to buy a four-page supplement in the Metro freesheet in London and other British cities urging readers to vote Leave.
The DUP's political opponents have queried the source of the CRC's money.
Sir Jeffrey Donaldson said: "I believe that they have raised their money legitimately and we were delighted to receive the donation from them for the Brexit campaign."
He told the Open Democracy website: "We have published the details of that, we have complied not only with the law but we have gone much further than we were required."
Richard Cook, a former Scottish Conservative, chairs the CRC.
He has rejected as laughable the suggestion that the group's funding is shadowy and denied some of it is foreign.
Sinn Fein has said voters in the General Election have a right to know the actual source of the donation.
Political gifts in Northern Ireland are kept confidential for fear of identifying donors, a legacy of the violent conflict.
The DUP has said the party voluntarily published details of the donor who helped fund the DUP's participation in the European referendum campaign, a step not taken by Sinn Fein which benefits from millions of pounds worth of US money.
DUP leader Arlene Foster accused the media of not subjecting other parties to the same examination of their finances.
Sinn Fein's Mairtin O Muilleoir has said the £282,000 cost of the advert was more than three times the amount spent by the DUP in last May's Stormont election.