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 - a publication not circulated in Northern Ireland.
DUP MP Sir 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.
Sir Jeffrey 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 Sir Jeffrey.
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."
Alliance leader Naomi Long said questions still remain over the DUP's Brexit expenditure.
She said the party should release a detailed breakdown of how the money was spent.
She also claimed the party only released the information because of public pressure.
"Would the DUP have campaigned as vigorously for Brexit had they not received a large donation to do so? How does that set against the previous allegations by Arron Banks that they were essentially charging a fee to join a Leave campaign? All of those questions need to be answered and answered openly," said Ms Long.
Mr Banks, a multi-millionaire who poured millions of pounds into the campaign for the UK to leave the EU, previously claimed that the DUP demanded money to back him.
However, the DUP denied the allegations made by Mr Banks in a book called The Bad Boys Of Brexit.
Ms Long said: "We still need full transparency so that people can make their own judgment as to whether parties are acting on their behalf as the electorate or on behalf of those with deep pockets and fat wallets."
She added: "I don't believe the DUP would have released details of their Brexit donor had they not come under pressure.
"They said in 2014 they would release all of their donor information and they were in favour of full openness and transparency; they didn't do it. They said it again in their 2016 manifesto; they didn't do it. It is only because of public pressure on this occasion that we have had any transparency.
"But this is only one donation. We now need to see the full list of donors to all political parties in Northern Ireland, not just the DUP."