A Scottish MP has written to the country’s Electoral Commission over concerns that pro-Unionist groups flouted the rules for online adverts in last week’s election.

It follows an openDemocracy report, published last week, which revealed that unofficial Unionist groups had spent tens of thousands of pounds on Facebook adverts calling for tactical voting in the Scottish parliament elections – without giving details of who paid for them.

The SNP’s Brendan O’Hara has asked the election regulator for a “comprehensive response”, saying the campaign adverts “may be contrary to election law and regulations”.

O’Hara says concerns centre around the tactics of third-party campaign groups, which , he claims, “almost never declare their sources of funding”.

In the lead-up to the election, pro-Unionist groups spent huge amounts on digital advertising. Election laws in Scotland state that adverts should include details of who paid for them. But many of the examples uncovered by openDemocracy failed to include sources of funding.

One group, called ‘Young Unionists’, spent more than £20,000 on Facebook ads, including more than £5,000 in the final days before the election. Tens of thousands of people were pushed to the VoteUnion tactical voting tool, but no name was attached to the campaign.

Meanwhile, a pro-Trump student group called ‘Turning Point UK’ spent £3,700 on Facebook ads in the week before the election.

Over just three days, at least 70,000 people saw one of its adverts, saying: “vote Conservative, Labour or Liberal to remake Scotland!” But no details were included about who paid for it.

Another campaign group, ‘Businesses for the Union’, spent more than £10,000 during the Scottish elections. Facebook data shows they were paid for by the deputy director of the Adam Smith Institute, Matthew Kilcoyne. However, the original source of funding is not known, because the institute does not declare its donors.

In his letter to the Electoral Commission, O’Hara also raised questions over whether some third-party groups may have worked together to coordinate their campaigns, which is regulated by strict electoral law.

The concerns have arisen from the fact that some pro-Unionist groups in Scotland are run by individuals who are very closely linked – including some who have worked on campaigns together in previous elections.

Writing to the Electoral Commission, O’Hara said: “I would be very grateful for a comprehensive response to these concerns at the earliest opportunity.

“It is vital the electorate has full confidence in the electoral process and that open democracy and accountability can be demonstrated.”

The MP, who used to sit on the Westminster parliament’s digital, culture, media and sport committee, previously called on the Electoral Commission to publish evidence surrounding secretive donations in the Brexit referendum.

The Electoral Commission did not confirm whether an investigation would be launched into the dark money ads in last week’s elections in Scotland. However, a spokesperson told openDemocracy: “We have received a letter from Mr O’Hara and will respond in due course.”