THE SNP faces the embarrassing prospect of a third MP withdrawing from the party whip after Westminster’s sleaze watchdog launched a full parliamentary investigation into the Nationalist’s conduct.

Phil Boswell, who admitted he had benefited from a tax avoidance scheme despite campaigning against them in the Commons, is being probed over whether he breached parliamentary rules on MPs declaring company directorships.

The office of Kathryn Hudson, Parliamentarythe Commissioner for Standards, said: “The commissioner has begun an investigation into Mr Boswell. The issue relates to the registration of interests.”

Last month, Ms Hudson received a complaint about the MP from Paul McGarry, a Liberal Democrat candidate for central Scotland at the Holyrood election, over his alleged failure to record that he was a director of a company, Boswell and Johnston Ltd, in the register.

The MP for Coatbridge, Chryston and Bellshill received an £18,309 interest-free loan from a former employer, “repayable on demand”, as part of his role as a contract manager in the energy sector.

He explained the payment was “common practice” in the industry, and the SNP has since confirmed the MP has been in touch with HM Revenue and Customs to discuss his tax position.

Companies House records show that Mr Boswell is the sole director of Boswell and Johnston, but his entry in the Commons register of interests fails to mention the company nor any interest-free loans or dividends he might have been paid through this firm.

The SNP is adamant that Mr Boswell “declared all interests in line with the rules, including all relevant income”.

Ms Hudson’s spokeswoman said she had to consider whether or not a complaint was within her remit and if there was sufficient evidence to launch a full inquiry. In this case, she said, “both the tests are met”.

Asked if the watchdog would personally interview Mr Boswell, she added that Ms Hudson would “conduct her inquiry in whichever way is appropriate” and this could involve a meeting with the MP and/or communication by email. She would seek to conclude her investigation “as soon as possible while being fair to all the parties concerned”.

Last month Mr Boswell was criticised for using the tax avoidance scheme.

Liberal Democrat peer Lord Purvis of Tweed said the parliamentary rules appeared to have been breached, in “not declaring personal gain when raising, hypocritically, tax evasion schemes in Parliament”.

The commissioner’s inquiry could have one of three outcomes: there is no case to answer, which would result in the matter being dismissed; there has been a minor breach of the Commons rules, which would include “rectification” by the MP and an apology; or there has been a serious breach, which would result in the matter being referred for action to the Commons standards committee.

In the latter case, temporary suspension from the Commons or even expulsion from it could follow.

Given the commissioner has now begun a full investigation into Mr Boswell’s registration of interests, political pressure for him to withdraw from the SNP whip could begin to grow.

In the eight months since the General Election, two SNP MPs have already voluntarily withdrawn from the party whip.

In September, Michelle Thomson, the MP for Edinburgh West, did so after she became the subject of a Police Scotland investigation into alleged irregularities with property deals.

In November, Natalie McGarry, the MP for Glasgow East, followed suit after the police were called in to investigate apparent discrepancies in the campaign fund of Women for Independence, of which the backbencher was a founding member.

Both MPs strongly deny any wrongdoing. If Mr Boswell were also to withdraw from the party whip, then the SNP’s parliamentary number at Westminster would, for the time being at least, have fallen from 56 to 53.

The Herald attempted to contact Mr Boswell the MP but there was no response.