SERIOUS questions are piling up over Scottish Conservative donations, the SNP has claimed, as David Mundell insisted he was “absolutely satisfied” that his party had complied fully with all election spending rules.

The escalating row centres on the role of the Scottish Unionist Association Trust[SUAT], which has given the Tories in Scotland nearly £320,000, and which is now being examined by the Electoral Commission, the elections watchdog, to see whether or not it is a permissible donor. Critics have branded SUAT donations “dark money”.

The SNP’s Pete Wishart has written to the commission to enquire about the extent of the SUAT’s property portfolio, which, he suggested, could disbar it from being a permissible donor.

The backbench MP has called on Ruth Davidson, the Scottish Conservative leader, to reveal if any of the SUAT donations were used in the 2017 General Election campaigns of Tory MPs John Lamont, David Duguid and Douglas Ross, all of whom took seats from sitting Nationalist candidates last year.

Asked on the BBC’s Good Morning Scotland programme if SUAT cash was used to fight seats in the General Election, Mr Mundell said: “The Scottish Conservatives have registered and appropriately declared and set out all the funding they have received from the SUAT and all other sources of funding.”

The Scottish Secretary made clear he had not personally received cash from the SUAT and stressed that he was confident that all money received during his time in politics had been “appropriately declared” and that the Scottish Conservatives had followed the rules "assiduously"; if anyone had evidence to the contrary, they should take it to the commission, he added.

But Mr Wishart insisted Mr Mundell’s responses had failed to provide a satisfactory explanation over whether or not the Scottish Tories had “used dark money in last year’s General Election to bankroll campaigns in key target seats”.

The Perth MP went on: "Transparency over party financing is absolutely essential in any healthy democracy; the public need to have a clear understanding of who helped put their elected representatives into office.

"As each day passes, more and more questions are piling up for the Scottish Tories; it is time for Ruth Davidson to break her silence and answer the questions that her colleagues are refusing to answer.

"Was dark money used in last year’s election campaign? And if it was, are the Tories satisfied that the election expense returns of John Lamont, David Duguid and Douglas Ross were correctly completed? And have David Mundell’s own election campaigns received money that originated from SUAT over the years?"

He added: "The Electoral Commission needs to use its powers to crack down on rule-breaking, cheating and dark money. Political campaigns have to be a level playing field to protect democracy; that is why this investigation is so serious and so urgently needed."