Judicial review proceedings are being issued against the Electoral Commission over its decision not to investigate the handling of the DUP's biggest ever donation.
The Good Law Project is seeking the review from London's High Court.
It relates to the Electoral Commission's decision not to investigate the £435,000 donation made during the EU referendum.
The bulk of the money was spent by the DUP on pro-Brexit advertising.
The Electoral Commission said it has carried out its enforcement duties to "the highest standards".
A spokesperson said: "We are disappointed that the Good Law Project continues to pursue this judicial review. They are using this particular case, and the restrictions imposed upon the commission by the Northern Ireland transparency laws, to fuel public mistrust where none is merited."
The donation was made by the pro-Brexit group the Constitutional Research Council (CRC).
On Wednesday BBC News NI reported that the CRC broke electoral law by failing to report the donation to the Electoral Commission.
Following an investigation, the CRC declared the 2016 donation and the commission found the source of the money was permissible.
The CRC is chaired by Richard Cook, a former vice chairman of the Scottish Conservative and Unionist Party.
The DUP did report the donation and has consistently insisted it complied with electoral law at all times.
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
In June, a BBC Northern Ireland Spotlight programme questioned whether DUP Treasurer Gregory Campbell had checked the permissibility of the donation from the CRC.
In August, the Electoral Commission announced it would not investigate the allegations contained in the film, having made what it said was "a thorough review of the programme".
The Good Law Project is a campaign group that works to uphold the rule of law. One of its central aims is to try and stop Brexit.
On Thursday, the organisation confirmed it was lodging proceedings at London's High Court challenging the Electoral Commission's decision not to take action over the CRC donation.
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 can not release information on any donations made before July 2017.
Jolyon Maugham QC, barrister and founder of Good Law Project, said: "It is of enormous concern that the DUP, who currently wield disproportionate power, appear so unconcerned about the source of donations, and that once again the Electoral Commission's regulation of the EU Referendum - the most important vote of our generation - appears to lack any kind of rigour."
A spokesperson for the Electoral Commission said: "The Good Law Project knows that the law currently results in a lack of transparency around past political donations in Northern Ireland. We are therefore unable to provide more complete information about the steps we took to fulfil our regulatory duty in this particular case. What we can say, however, is that we fulfilled this duty to the highest standard."