The deputy leader of the Scottish Conservatives has called on a former party candidate to say who made a £435,000 donation to a pro-Brexit group during the Brexit referendum.

Jackson Carlaw, the party’s second most senior figure, said Constitutional Research Council (CRC) chair Richard Cook should reveal the names of the donors who gave money to the organisation.

It emerged after the referendum on European Union membership that the CRC had donated nearly half a million pounds to the pro-Leave Democratic Unionist Party (DUP) in Northern Ireland.

However, the DUP was under no obligation to name the CRC as the source of cash, most of which was spent during the referendum on an advert in a newspaper that is not sold in Ulster.

The CRC is itself shrouded in mystery and has only confirmed that Mr Cook is its chair.

Mr Cook, a businessman who lives in East Renfrewshire, has declined to say publicly where the CRC got its money, or name the members of its executive committee. The CRC is not part of the Conservatives.

The Scottish Tories have also faced questions about the CRC as Mr Cook is a former vice-chair of the party. e stood as a Holyrood by-election candidate in Glasgow Cathcart in 2003 and fought the same seat four years later.

He also tried to topple Labour heavyweight Jim Murphy in Eastwood in 2010, but fell short by over 10,000 votes.

HeraldScotland:

Picture: Carlaw (left) on the campaign trail with Cook (right) in 2008

Mr Carlaw, who will stand in for Davidson later this year when she goes on maternity leave, helped campaign for Mr Cook in his last campaign.

Speaking to the Herald on Sunday, Mr Carlaw downplayed his political links to Mr Cook and said he had not had any “real” dealings with him since the 2010 campaign.

“I’ve had no substantive conversation with Richard since. I’ve followed with just as much interest and bewilderment, at times, what he has done since.”

Asked if Mr Cook should be transparent about the identity of CRC donors, he said: “From my perspective, yes.”

SNP MP Brendan O’Hara has called for Mr Cook to give evidence to Westminster’s Digital, Culture, Media and Sport committee, which has an ongoing inquiry into aspects of the Leave campaign.

Fellow Nationalist MP Pete Wishart called on the Electoral Commission to get a grip on the “dark money” row and said:

“Anonymous donations were supposed to be a thing of the past. Cook was behind the controversial £435,000 donation to the DUP during the EU referendum. With at least three quarters of it spent outside of Northern Ireland, Cook and the DUP have serious questions of how the CRC cash was used.”

A spokesman for Mr Cook said the Electoral Commission knows the identities of the CRC donors and has verified their eligibility.