The DUP is under renewed pressure to reveal the source of a major donation received during the EU referendum last June.
The sum of £435,000 was given by the Constitutional Research Council, a little-known Great Britain-based group of pro-union business figures.
Part of it was used to buy a four-page supplement in the Metro freesheet in London and other British cities urging readers to vote Leave.
Sinn Fein Assembly member Mairtin O Muilleoir said: "The Constitutional Research Council has no legal status, publishes no membership list, has no public presence and has failed to make public where it got the money.
"The DUP has refused to identify their dealing with the Constitutional Research Council and where the dark money came from.
"Voters have a right to know from the DUP where the actual source of the donation is."
Political gifts in Northern Ireland are kept confidential for fear of identifying donors, a legacy of the violent conflict.
DUP leader Arlene Foster has defended her handling of questions around the donation and accused the media of not subjecting other parties to the same examination of their finances.
Mr O Muilleoir said the £282,000 cost of the advert was more than three times the amount spent by the DUP in last May's Stormont election.
A statement from the Electoral Commission said: "We have consistently called for increased transparency on how political parties are funded in Northern Ireland.
"Such a move will only help to increase public confidence in the democratic process."
Meanwhile, the cross-community Alliance Party said a second referendum should be held on the terms of the UK's exit from the EU.
South Belfast General Election candidate Paula Bradshaw said: "We recognise Article 50 has been triggered.
"However, people's attitudes have markedly changed as the truth about Brexit and just how harsh it will be for our economy and public finances, as well provision of health services and education, becomes clear."