Prime Minister Theresa May has been urged to investigate donations to the Scottish Conservatives amid claims they were trying to evade scrutiny.
The elections watchdog is investigating whether cash from the Scottish Unionist Association Trust complied with rules.
SNP Westminster leader Ian Blackford claimed the Tories were “systematically shielding their donations” from view.
But Mrs May insisted that all donations to her party were “accepted and declared in accordance with the law”.
The Scottish Unionist Association Trust (SUAT) has now published information which Mr Blackford had asked for about the identity of its trustees, and said it was “in dialogue with the Electoral Commission”.
A spokesman for the Scottish Tories said the party “always works closely with the Electoral Commission on the permissibility of donors”.
An investigation by the Ferret website found that the SUAT donated £318,876 to the party between April 2001 and February 2018.
Under Electoral Commission rules, groups which make political contributions of more than £25,000 have to register with the watchdog, and report any donations in excess of £7,500.
The SUAT is not listed as an “unincorporated association” on the Electoral Commission’s register, and the Commission said it was investigating whether the trust had “complied with their reporting requirements as set out in law”.
The issue was raised at the weekly session of questions to the prime minister by Mr Blackford. He highlighted money from the trust which was given to campaigns for MSP Jackson Carlaw and MPs David Duguid and Douglas Ross.
Mr Blackford said: “There is no information available about who the people are who currently manage the trust, no public accounts indicate who its donors are, or what assets it holds.
“The BBC has revealed that the former vice chairman of the Conservative Party in Scotland, Richard Cook, was behind the DUP’s £435,000 donation during the EU referendum and has a trail of involvement in illegal activity and foreign money.
“I’m giving the PM the chance to tell us what checks the Scottish Tory party had in place before accepting such large donations, and will she investigate the links between the party and the trust and promise to publish a list of all donations and donors?”
Mrs May replied: “All donations to the Scottish Conservative Party are accepted and declared in accordance with the law, and the Scottish Conservative Party works in accordance with the Electoral Commission to make sure that is all done properly.”
Following these exchanges, the SUAT contacted the BBC with responses to Mr Blackford’s questions about its trustees and background.
The group said it was set up in 1968 with assets from the old Scottish Unionist Association, primarily from the sale of property. The money was invested and the proceeds made “available to further the aims of the Scottish Conservative and Unionist Party”.
The group – which is chaired by Robert Miller-Bakewell – said all of its UK tax liabilities “have been and continue to be met in full”.
A spokesman said it would be “inappropriate” to comment further while “dialogue” with the Electoral Commission continues.
Although few of the group’s donations listed by the Electoral Commission exceed £7,500, the combined total for 2017 was in excess of £35,000. In 2015, another Westminster election year, the group is logged as having donated more than £34,000 to various local Tory groups – including £8,383.44 to the party in Dumfries and Galloway.
The donations to Mr Duguid and Mr Ross, in 2017, were each of £7,500. More than 100 donations have been logged since 2001, working out at an average of just over £3,100.
The Ferret investigation said the funding was “legal”, but raised questions about why so little information was in the public domain about the group.
Speaking on the BBC’s Good Morning Scotland programme, Mr Duguid – who won the Banff and Buchan seat from the SNP in the 2017 election – said he “was more focused on winning the election than where any donation was coming from”.
He said he “wouldn’t call it dark money”, insisting that the donations “went through the appropriate process and approvals”.
A Scottish Conservative spokesman echoed the prime minister’s words, saying: “The party has declared all donations from the Unionist Trust and always works closely with the Electoral Commission on the permissibility of donors.”