The Democratic Unionist Party (DUP) accepted a further £13,000 donation from a pro-Brexit group in the months after the EU referendum, documents have confirmed.
The Constitutional Research Council (CRC) had previously donated £435,000 to the DUP during the 2016 Brexit referendum campaign.
The bulk of the £435,000 was spent by the DUP on pro-Brexit advertising.
The DUP said it has complied with electoral law at all times.
The party did not comment on how it spent the £13,000 donation but said it used donations to “further the cause of unionism at home and abroad”.
The details on the latest CRC donation are contained in internal Electoral Commission documents published by the campaign group the Good Law Project.
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.
BBC News NI contacted Mr Cook about the £13,000 donation to the DUP but he was unavailable for comment.
The names of those who donated the money to the CRC have never been released.
Donor laws in Northern Ireland state that the Electoral Commission cannot publish any donations made before July 2017.
In February 2017, the DUP confirmed it received a £435,000 donation from the CRC as part of the EU referendum campaign.
Most of that money was spent on the Brexit campaign, including a four-page “Vote To Leave” advertisement in the Metro newspaper, which is available in London and other cities but not in Northern Ireland.
The DUP reported the donation to the Electoral Commission but BBC News NI previously revealed that the CRC was fined £6,000 by the commission for failing to report the donation.
Following an investigation, the CRC declared the donation and the commission found the source of the money was permissible.
However, the latest batch of Electoral Commission documents confirm that the CRC gave the DUP a further £13,000 after the EU referendum.
A donation of £6,000 was made in October 2016 and a further £7,000 was given in March 2017.
Both donations were correctly declared to the Electoral Commission.
The details of the £13,000 donation were contained in an assessment by the Electoral Commission of allegations made in a BBC NI Spotlight programme.
It examined whether there was a common plan between the DUP and the referendum campaign group Vote Leave.
Last August, the Electoral Commission announced it would not investigate the allegations contained in the programme, having made what it said was “a thorough review of the programme”.
Speaking to the Open Democracy website, some MPs have called on the Electoral Commission to re-open its investigation into the connections between Vote Leave and the DUP.
Jolyon Maugham, from the Good Law Project, said it was “inevitable” that the Electoral Commission would need to re-examine donations to the DUP during the referendum campaign.
He added: “It’s extraordinary that – almost three years on – real questions remain.”
A DUP spokesperson said donations received by the party were reported to the Electoral Commission “in accordance with our legal obligations”.