Borders MP John Lamont at last year's general election count in Kelso.

That assertion comes after it emerged that basic details about the trust, the source of a fifth of all Scottish-registered donations to the Conservatives in the months leading up to last year’s June 8 vote, were not in the public domain.

An investigation by Edinburgh-based media co-operative the Ferret last month found that there was no information available about the people managing the organisation and no public accounts to indicate who its donors are or what assets it holds.

There was no official base listed for the trust either, though three different addresses – in Glasgow, Edinburgh and Whiterigg near Melrose, used by Borders Tories to host a drinks party with Horsham MP Jeremy Quin, attended by Mr Lamont, in May – had been cited in official records.

Following those revelations, Ian Blackford, leader of the Scottish National Party group in the House of Commons, claimed the Conservatives were systematically shielding their donations from public scrutiny and accused the trust of funnelling “dark money” into the party.

During Prime Minister’s questions last week, Ross, Skye and Lochaber MP Mr Blackford said: “The trust has donated £319,000 to the Scottish Conservatives, yet there is no information available about who the people are who currently manage the trust and no public accounts to indicate who its donors are or what assets it holds.”

Mrs May told him that all donations to the Scottish Conservatives were accepted and declared within the law.

“The Scottish Conservatives worked with the Electoral Commission to make sure that it is all done properly,” she said.

The trust has since published a statement giving an address in Mosspark Boulevard, Glasgow and a list of its trustees, including chairman Robert Miller-Bakewell, owner of Whiterigg in Melrose and also deputy chairman of Scottish Borders Conservatives and U nionists.

However, question marks remain over the unincorporated association, responsible for handing out nearly £29,000 in donations to Scottish branches of the Tory party in the first two quarters of 2017, including nearly £12,000 to the Berwickshire, Roxburgh and Selkirk branch ahead of Mr Lamont’s general election victory a year ago last month, ousting the SNP’s Calum Kerr.

Speaking to the Southern this week, Mr Lamont said that all donations from the trust had been declared by the Scottish Conservative Party and made public.

“The donations were all permissible donations and properly declared with the Electoral Commission,” Mr Lamont said.

He added that his party would continue to work closely with the commission and comply with rules surrounding political donations.

Under commission rules, unincorporated associations that donate more than £25,000 in a calendar year must register with the body and report gifts of £7,500 or more.

However, the trust is not listed in the commission’s register of unincorporated associations, so the regulator has vowed to investigate.

In a statement issued last Wednesday, the trust said it was formed in 1968 to take over the assets of the then Scottish Unionist Association, primarily derived from property sales.

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.”