SNP politician Pete Wishart has demanded Theresa May come clean about the circumstances leading up to the £70,000 fine imposed on the Tory party by the Electoral Commission last month.

The watchdog issued the record penalty and reported the the Conservative party’s former treasurer to police after a year long investigation uncovered significant failures in accounting. More than £275,000 of campaign spending at three byelections had not been properly declared.

Party officials, the Commission said, had partially declared some items as national spending, when they should have been included in the local spending limits, giving the Tories an “financial advantage over opponents”.

Central to those investigations, were the Battlebus, where activists were transported in to campaign in key marginal seats, and reports of a team of senior party officials sent to help organise in South Thanet, where the Tories were fighting off the then Ukip leader, Nigel Farage.

The expenses for both were declared as part of the party’s overall national spend.

In his letter to the Prime Minister, Wishart, the SNP’s shadow leader of the House of Commons, urged her to be “fully transparent” and explain who knew what and when. The Electoral Commission is only authorised to examine national spending, it is the police who are responsible for looking into allegations of excessive spending on local campaigns.

Detectives from forces all over England are currently investigating around 20 MPs and agents. Around a dozen police forces have passed files to the Crown Prosecution Service.

Wishart, who asked May about the fine during a recent bout of Prime Minister’s Questions said: “Clearly this is not going away and the Tories must answer not just my questions: there has to be total clarity about this whole sorry tale.

“As a previous Tory leader once said – ‘the best disinfectant is sunshine’ – so let’s shine some light on this mess.’’ In the letter Wishart tells the Prime Minister: “It is simply inconceivable that a political party as well resourced as the Conservative Party was unaware of our electoral rules and far from being an ‘administrative error’ do you agree that this is at best wilful negligence and at worst electoral fraud?”

A Conservative Party spokesman said: “The Conservative Party has complied fully with the Electoral Commission’s investigation since it began more than a year ago and will pay the fines they have imposed.

“This investigation and these fines relate to national spending by CCHQ, and the Conservative Party’s national spending return for the 2015 general election. As we have consistently said, the local agents of Conservative candidates correctly declared all local spending in the 2015 general election.

“CCHQ accepted in March 2016 that it had made an administrative error by not declaring a small amount constituting 0.6 per cent of our national spending in the 2015 election campaign. This error was subsequently corrected and the Party has since improved its accounting practices, reporting structures and staff guidance. Even taking this into account, the Conservative Party still considerably underspent the statutory national spending limits for the 2015 general election.

“Political parties of all colours have made reporting mistakes from time to time. The Labour Party and Liberal Democrats both failed to declare sums of money which constituted a larger proportion of their national expenditure in the 2015 general election. Both have been fined by the Electoral Commission, and the Liberal Democrats are also under police investigation.

“This is the first time the Conservative Party has been fined for a reporting error.”