The Democratic Unionists have revealed that their £425,000 Brexit campaign was bankrolled by pro-Union group the Constitutional Research Council.
The party had been under mounting pressure to make public the name of the mystery donor who enabled it to promote its Leave message on a UK-wide basis, including the purchase of an expensive four-page Vote To Leave EU advertisement in the British Metro newspaper.
The paper is available in London and other cities but not in Northern Ireland.
DUP MP Jeffrey Donaldson said: "The donor is the Constitutional Research Council, it is a pro-Union unionist organisation based in Great Britain.
"It has been involved in Scotland, for example supporting unionism in Scotland, and it approached the DUP to support our campaign during the referendum because it supports unionist causes in the United Kingdom."
The CRC is a group of pro-Union business figures chaired by former vice chairman of the Scottish Conservative Party Richard Cook.
The names of donors in Northern Ireland are automatically withheld due to Troubles-era rules which sought to protect their security.
That had prompted speculation that prominent Leave campaigners were using the DUP to support the Leave campaign without the need to publish their names.
Mr Donaldson rejected that suggestion.
The Lagan Valley MP said the CRC had donated to the DUP because the main Leave campaign had already reached the limit of its spend.
The identity of the DUP's Brexit donor had become an issue in the ongoing Stormont Assembly election, with it facing calls from political rivals to reveal all.
"We were delighted we were able to raise that amount of money," said Mr Donaldson.
He outlined the details hours before the Electoral Commission published details of campaign spending on the EU referendum.
The MP told Radio Ulster: "We wanted to be involved in the referendum at a national level. We recognise that the population of Northern Ireland is 3% of the United Kingdom.
"This referendum wasn't going to be won or lost in Northern Ireland, it was going to be won or lost on a national basis, and that's why the DUP, being a unionist party, decided to participate in the national campaign and we registered as such and, after we registered, we were able to raise this money and spend it all on campaigning both on a national level and indeed we spent some of it in Northern Ireland."
Social Democrats want guarantees law not being abused
The Social Democrats has, meanwhile, called on the Government and Ireland's ethics watchdog to offer guarantees Northern Ireland's anonymous donor laws are not being exploited to the advantage of political parties in the Republic.
Co-leader and Kildare North TD Catherine Murphy said political parties in Ireland are subject to very strong regulations on donations.
"The UK allows unlimited donations from donors, with medium to large donations are made public.
"However, there exists an exemption for political parties in Northern Ireland, where parties can receive unlimited amounts of money without having to say publicly who gave it."
She said this includes donations from Irish citizens, companies, trades union and political parties in and from the Republic of Ireland and this information is kept out of the public.
"Being able to trace the sources of funding for political parties is of vital importance to democracy and transparency. This is crucial to maintaining trust in our system, especially where parties are able to accept donations from Irish citizens and businesses."