A group accused of funnelling "dark money" to the Scottish Conservatives has been fined for an unlawful £100,000 gift it gave to the party in the run-up to the 2016 Holyrood election.

The Irvine Unionist Club, an unregistered donor, was fined £400 by the Electoral Commission for failing to notify the UK's electoral body about the six-figure gift.

Electoral law states that unincorporated associations (UAs) such as the Irvine Unionist Club which donate more than £25,000 in a calendar year must register with the commission and report gifts of £7500 or more.

The Scottish Parliament election in May 2016 saw Ruth Davidson's party push Scottish Labour into third place, after nearly quadrupling its election spending from £275,000 in 2011 to £980,000.

The gift from the Irvine Unionist Club, which was received by the Scottish Tories in March 2016 and officially reported the next month, amounts to more than 10% of the party's election spending total.

It comes as another Conservative-linked UA, the Scottish Unionist Association Trust (SUAT), remains under investigation by the Electoral Commission over whether it too breached the rules.

SUAT has donated nearly £320,000 to the Scottish Tories since 2001, and an investigation by STV News in July found that a fifth of all declared donations to the party in the run-up to 2017's snap general election came from the trust.

The probe is looking into the group's status as an "exempt trust" - meaning it currently does not need to report donations in the usual way.

The SNP said the penalty imposed on the Irvine Unionist Club is "probably just the first" to hit Tory-linked groups and claimed the "dark money net is closing in" on the party.

Scottish Labour is calling on Ruth Davidson to "come clean" about Tory donors, who it says are "not playing by the rules and are seeking to undermine the democratic process".

But the Conservative party said it was not under investigation, and added that the trustees at the club had "accepted that they were at fault" and paid the fine.

The Tories nearly doubled their seat total in 2016, going from 17 MSPs to 31 to oust Kezia Dugdale's Labour party as the main opposition at Holyrood.

The SNP won the most seats in the election but, partly due to the strong Conservative showing, failed to replicate the historic majority it won in 2011.

SNP MP Pete Wishart said: "The dark money net is now closing in on the Tories as their dodgy and cavalier financial dealings become further exposed and punished.

"This is probably just the first of many examples where the Tories will be found short of what is permissible by the Electoral Commission.

"Last week I wrote to the Electoral Commission for an update on my complaint about the transfer of property to the Scottish Unionist Association Trust in flagrance of the Commission's rules on exempt trusts under section 162 of the Political Parties, Elections and Referendums Act 2000.

"I hope that the Electoral Commission now make speedy progress with this investigation."

He added: "Some £318,000 of unaccountable money has been swirling about in Conservative coffers supporting a number of candidates and MPs."

Scottish Labour MSP Neil Findlay said: "This decision raises yet more questions about Tory dark money donations.

"The public has a right to know who is bankrolling these Tories and what these secretive donors expect in return for their large sums of money.

"The Electoral Commission verdict clearly shows Tory donors are not playing by the rules and are seeking to undermine the democratic process.

"It is now time for Ruth Davidson to come clean on who has been funding her party and why - anything less would be an insult."

A spokesman for the Scottish Conservatives said: "The Electoral Commission has investigated the donation, and has concluded that the [Irvine Unionist Club] was not exempt in terms of the 2000 Political Parties Act's reporting requirements.

"The trustees have accepted that they were at fault in failing to register the donation, and have paid the £400 fine.

"The Conservative Party was not investigated nor subject to any fine."

Bob Posner, political finance director at the Electoral Commission, said: "The reporting requirements are clear, so it is always disappointing when parties and campaigners fail to comply.

"It's vital that voters are given an opportunity to see accurate and full reportable data on what parties and campaigners spend money on in order to influence them at elections.

"This provides transparency in the political finance system and is open for anyone to scrutinise.

"The Commission will continue to enforce these requirements on all parties and campaigners to ensure voters have the information they need."