A fifth of all declared donations to the Scottish Conservatives in the run-up to 2017's snap general election came from a trust accused of funnelling "dark money" into the party, STV News can reveal.
In the first two quarters of 2017, nearly £29,000 of donations recorded by Scottish branches of the Tory party came from a group called the Scottish Unionist Association Trust (SUAT).
Electoral Commission data shows the trust accounted for 20% of all Scottish-registered donations to the Conservatives in the months leading up to last year's June 8 vote.
SUAT has been at the centre of a row over party funding, with the SNP raising questions about transparency of donations.
The party's Westminster leader Ian Blackford raised the matter with Theresa May at Prime Minister's Questions on Wednesday, and another SNP MP has written to Ruth Davidson about the trust.
Listed as an unincorporated association (UA), SUAT has donated around £319,000 to the Scottish Tories since 2001, an investigation by The Ferret revealed last week.
The Conservatives maintain that every donation declared from the trust has been done in accordance with Electoral Commission rules and that there is no wrongdoing.
Earlier on Wednesday afternoon, SUAT disclosed its current seven trustees and address following pressure to make the information public.
Last June's general election saw Theresa May's party lose its parliamentary majority but gain 12 Westminster seats in Scotland.
The Scottish Conservatives claimed the scalps of key SNP figures like Alex Salmond and Angus Robertson as they took their number of MPs from one to 13.
Following the Prime Minister's announcement of a snap poll on April 18, 2017, declared donations to the Conservatives across the UK ballooned from £5.5m in the first quarter of the year to £25.2m in the second quarter.
In Scotland, as when the party surged to second place in the 2016 Holyrood election, the Conservatives bolstered their campaigns in key seats with donations from SUAT and other individuals and groups.
In the year leading up to the 2017 snap poll, the trust poured nearly £12,000 into the Berwickshire, Roxburgh and Selkirk branch of the Conservatives - where Tory John Lamont ousted the SNP's Calum Kerr last June.
It also sent a £7500 gift to the party's Moray branch, where the SNP's then-Westminster leader Angus Robertson was to be toppled by Douglas Ross, and the same amount to Aberdeenshire, where the party achieved a hat-trick of wins for David Duguid, Andrew Bowie and Colin Clark.
Under Electoral Commission rules, UAs that donate more than £25,000 in a calendar year must register with the body and report gifts of £7500 or more.
In the first half of 2017 alone, the SUAT paid out £28,906 in donations to Scottish Tory branches along with an £825 payment to the Conservative party centrally.
But the trust is not listed in the commission's register of unincorporated associations, which the regulator has vowed to investigate.
The Ferret uncovered title deeds showing a property in Glasgow had been transferred into SUAT's ownership from another Conservative-linked trust in 2014 - a building the Scottish Tories confirmed they rented from the trust as its office in the city.
SUAT's status as an "exempt trust" means that the usual requirements under British electoral law - that political parties must check donations are from permissible sources before accepting them - do not apply.
However, under Electoral Commission rules, a trust can only be classed as exempt if no property has been transferred to it since the introduction of new legislation in 1999.
The electoral body has said it will look into SUAT's exempt status.
The Scottish Conservatives told STV News the party will cooperate fully with the Electoral Commission on this matter as it does in all others.
A spokesman said: "All our donations are registered and declared appropriately in line with Electoral Commission rules.
"Questions for the Scottish Unionist Association Trust will have to go to them."
SUAT confirmed in its statement on Wednesday that it was "formed in 1968 from assets of the (then) Scottish Unionist Association, primarily sales of property asset".
It continued: "It invests those assets and makes the proceeds available to further the aims of the Scottish Conservative and Unionist Party.
"All UK taxation liabilities have been and continue to be met in full."
A spokesman for the trust added: "We are in dialogue with the Electoral Commission and it would be inappropriate to say more whilst that dialogue continues."
SUAT named its address as in Mosspark Boulevard in Glasgow's south side and its current chairman as Robert Millar-Bakewell, a former member of the Scottish Conservative executive.
The other six trustees are former party chairwoman Kim Donald; former Tory conference convener Patricia McPhee; Renfrewshire branch treasurer Sheila Fulton; ex-MP Peter Duncan; ex-Westminster candidate Frank Spencer-Nairn; and John Duncan.
SNP MP Martin Docherty-Hughes wrote at the weekend to Scottish Tory leader Ruth Davidson on the issue of "dark money", asking about what checks the Conservatives have in place for large donations from UAs.
Blackford quizzed May on this matter in the House of Commons earlier, accusing the Scottish Conservatives of "systematically shielding their donations from public scrutiny".
He also asked about Richard Cook, the former vice chairman of the Scottish Tories and the man behind a controversial £435,000 donation to the DUP during the EU referendum.
The gift, its source protected from scrutiny by Northern Irish campaign finance laws, was used to buy pro-Brexit adverts, including a £280,000 four-page wraparound ad in the Metro newspaper.
Mr Cook also used an opaque UA called the Constitutional Research Council (CRC), of which he is the only known member, to make the payment.
The £435,000 sum paid to the Northern Irish party - the largest single donation in British political history - was used for advertising not in Northern Ireland, but on the UK mainland.
The DUP was a registered campaigner in the EU referendum, therefore entitled to campaign throughout the UK, however any coordination with the Vote Leave campaign may have been illegal.
Blackford also raised reports that the Electoral Commission is to separately accuse Vote Leave of illegal coordination with a student organisation during the referendum.