Secretary of State James Brokenshire has been urged to reveal the mystery donor who bankrolled the DUP's pro-Brexit adverts in the British press.
Green Party leader Caroline Lucas has tabled two written questions in Westminster calling on the secretary of state to publish details of what Northern Ireland parties spent during last year's EU referendum campaign.
Days before the UK voted to sever its ties with Brussels last June, the DUP sponsored a wraparound advert in the Metro newspaper. It has been estimated that the ad, which was seen all over Britain but not in Northern Ireland, cost in the region of £250,000.
It is thought the DUP sponsored the ad so the mystery backer's identity would remain a secret due to rules in the north which forbid the publication of donor's identities.
When pressed on the matter during last Thursday night's UTV leaders' debate, Arlene Foster said: "They (the funds) are from an organisation in England that wants to see the union kept and make sure we can have a United Kingdom because it was a national vote..."
The DUP leader subsequently confirmed to the BBC that her party received a donation but that she could not recall the sum.
Despite Mr Foster's assurance that she would say where the money came from, the identity of the organisation remains secret.
The DUP was expected to face further media scrutiny about the matter at its manifesto launch in Belfast yesterday, however, in an unprecedented move the party refused to take questions from journalists.
Last night Ms Lucas told The Irish News that the "extremely large secret donations" to the DUP raised serious questions over the funding of the Leave campaign.
"Transparency must be at the heart of a fully functioning democracy and it’s crucial that the government publish this list of donors to the Northern Irish parties immediately so the public can see what was really going on," the Green MP said.
In two parliamentary questions tabled yesterday and due to be answered on Thursday, Ms Lucas calls on Mr Brokenshire to publish all donor information held by the regional Electoral Commission from 2014 onwards.
She also calls on the secretary of state to make a statement on the controversy.
The second question urges Mr Brokenshire to publish a breakdown of spending by registered parties in Northern Ireland during the EU referendum campaign.
Sinn Féin last night called on its former partners in government to tell the electorate who the mystery donor is.
Nuala Toman said the DUP had yesterday talked of its commitment to increase transparency and accountability.
"Their mishandling of the RHI scandal damaged public confidence in the political institutions – in order to restore some of that confidence the DUP should release this information now," she said.
"By promoting a pro-Brexit stance the DUP were not acting in the best interests of the people of the north and they continue to do so by insisting that we are dragged out of the European Union against our wishes."
Last year leading Brexiteer Arron Banks claimed the DUP asked him for £30,000 a month over four months to back his Leave.EU campaign. Mr Banks also claimed the DUP eventually got paid £50,000 a month for backing Vote Leave, the official pro-Brexit campaign. The DUP has denied the claims.
