The DUP is under intense pressure to reveal the source of a large donation received during the referendum on Britain’s EU membership last June.

In February of this year, the DUP confirmed that it received a donation of more than half a million euro from a group of business people led by a Scottish Conservative party member, the ‘Constitutional Research Council’ (CRC). It passed almost all of it on to help fund the pro-Brexit campaign in Britain.

Two days before the Brexit referendum last June, the Metro freesheet in London and other British cities came wrapped in a hugely expensive, four-page glossy propaganda supplement urging readers to vote Leave.

It was paid for by the DUP, even though Metro does not circulate in Ireland.

The Metro wraparound cost a staggering 282,000 pounds (330,000 euro) - thought to be the biggest single campaign expense in the history of Irish politics. It dwarfs the 90,000 pounds (106,000 euro) the DUP spent on its entire campaign for the previous month’s assembly elections.

The DUP eventually admitted that this spending came from the donation of 425,622 pounds (530,000 euro). It has repeatedly refused to identify who provided the cash originally.

It is thought that the party was used as a front because, unlike Britain, there is no legal requirement for large party donations in the North of Ireland to be made public.

There are possible links between the cash and the Saudi Arabian royal family and the Saudi intelligence service through the head of the CRC, former Scottish Conservative parliamentary candidate Richard Cook.

Efforts to unearth the original source of what Sinn Fein describe as ‘dark money’ have been rebuffed by both the DUP and the CRC.

The DUP’s Brexit campaign manager, Jeffrey Donaldson, has said that the party did not need to know the true source of the money, a statement which appeared in violation of electoral law.

Mr Donaldson has also said that he couldn’t remember why his party choose to spend more than 32,000 pounds during the Brexit campaign on a controversial data analytics company linked to Donald Trump.

It paid 32,750 pounds to Aggregate IQ, a data analytics company based in British Columbia, Canada, according to accounts filed with the Electoral Commission. This money was to target voters on social media during the Brexit referendum campaign.

It is still unclear why the DUP decided to spend an amount more than half its budget for the 2015 general election with a little-known data analytics company in Canada, and Donaldson has provided no answers.

The firm has very close ties to Cambridge Analytica and hedge fund billionaire and Donald Trump-backer Robert Mercer. Cambridge Analytica has also been linked to the use of ‘big data’ psychological programming techniques to help swing the US presidential election in Trump’s favour.

Speaking on BBC’s The View, Arlene Foster said the controversy was an attempt to distract from “real issues”

She claimed she knew who the source donors were -- British businessmen, she said -- but refused to identify them. She said she was “satisfied that the people who gave it had every right to give the donation”.

“We have answered all the questions the Electoral Commission have asked us. It is satisfied we have done everything in accordance with the law and I am satisfied,” she said.

Sinn Fein has met with the Electoral Commission to discuss the unusual donation and campaign spending.

“Most of the money, 282,000 pounds, was spent in London where the DUP have no presence, on a pro-Brexit wrap-around in the Metro freesheet,” said former Sinn Fein finance minister Mairtin O Muilleoir.

“Putting this in context that was more than three times the amount spent by the DUP in last May’s Assembly election,” he added.

“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,” said Mr O Muilleoir.

The party’s election candidate in west Belfast, Paul Maskey, said there are “serious public concerns” about the donation.

“With allegations of involvement of individuals linked to Saudi Arabian intelligence it is vital that the public gets answers as it is illegal to receive election donations from a foreign government,” he said.