NICOLA Sturgeon and her husband will be forced to give evidence under oath in the Holyrood inquiry into the Alex Salmond affair, the Scottish Parliament has confirmed.

The First Minister’s deputy John Swinney, the Scottish Government’s most senior official, permanent secretary Leslie Evans, and Mr Salmond himself will also have to take an oath.

MSPs on the inquiry, which is looked at how the Government botched a misconduct probe into Mr Salmond and left taxpayers with a £500,000 legal bill, discussed the potential for oath-taking last week.

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After days of behind-the-scenes deliberations, the committee has now decided it will require witnesses to take an oath, effectively threatening them with prison if they refuse or lie.

In an updated approach published this morning, it said: "The Committee has agreed that it will administer an oath/solemn affirmation for witnesses as a matter of course but that it will review this position in the case of any vulnerable witnesses."

Under rarely used powers in the 1998 Scotland Act, people can be required to take an oath before giving evidence at Holyrood and jailed for three months or fined £5000 if they refuse.

Knowingly making a false statement under oath is punishable by up to five years in jail.

The committee’s initial witness list includes SNP chief executive Peter Murrell, who has been married to Ms Sturgeon since 2010, her chief of staff Liz Lloyd and Mr Salmond’s former chief of staff Geoff Aberdein.

The committee, which starts its oral evidence sessions in mid-August after a summer of written evidence gathering, also also wants to quiz the two permanent secretaries who served Mr Salmond, Sir John Elvidge and Sir Peter Housden, and the current Lord Advocate James Wolffe QC.

Ms Evans is expected to be the first witness.

The committee’s focus is the Scottish Government’s in-house probe into two complaints of sexual misconduct made against Mr Salmond in 2018 by two female civil servants.

After the inquiry’s existence became public in August that year, Mr Salmond resigned from the SNP after 45 years as a member in the resulting furore.

He then launched a crowdfunded judicial review at the Court of Session to have the probe’s findings struck down.

In January 2019, the Government admitted in court its probe had been unfair, unlawful and “tainted by apparent bias” because the investigating officer was in prior contact with the complainants, leaving taxpayers with a £500,000 bill for Mr Salmond’s legal costs.

It then emerged that Ms Sturgeon maintained contact with Mr Salmond while he was being investigated by her officials, leading to accusations she broke the Scottish ministerial code.

Two MSPs on the committee last week said witnesses should give evidence on oath to ensure accuracy.

Tory MSP Donald Cameron, an advocate, said “sworn evidence, evidence on oath” would help bring out the most accurate evidence.

He said: “We need the opportunity to test both its credibility and it’s veracity, not least because we’re likely to get conflicting versions of events, and there will be disputed areas of facts.

“Secondly, we are dealing with very serious matters involving the highest echelons of the Scottish civil service and the conduct of very senior ministers past and present.

“Taking sworn evidence underscores the gravity of the subject matter of this inquiry and will ensure we get the best possible evidence.”

Liberal Democrat MSP Alex Cole-Hamilton added: “I’d like to reiterate Donald Cameron’s request to have witnesses heard on oath. I think that this is a very important consideration. We will hear conflicting stories and I think it’s vital we have confidence in what we as a committee hear.”

The SNP has four of the nine seats on the cross-party committee. 

Mr Salmond has already given evidence under oath at his separate criminal trial in March, at which he was acquitted of 13 charges of sexual assault.

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Ms Sturgeon has already pledged to give evidence to the committee if she is called as a witness, saying in January 2019: "As First Minister I don't consider it optional to me whether or not I appear before parliamentary committees, that is a part of my job and a part of my responsibility."

After he was cleared on all counts at the High Court in Edinburgh, Mr Salmond and his allies suggested he had been the victim of a politically motivated plot to destroy him.

Ms Sturgeon has dismissed the conspiracy theory as a “heap of nonsense”.

The committee will examine how the Government probe was bungled, the judicial review, and whether Ms Sturgeon’s behaviour broke the ministerial code.

Mr Salmond is currently writing a book about the “nightmare” of his prosecution and trial.

A Scottish Government spokesperson said: "We welcome the opportunity which the parliamentary inquiry will bring to address issues which have been raised, and we will not pre-empt that process.”