The inquiry was launched in 2021 but ramped up in April as former chief executive Peter Murrell, Nicola Sturgeon's husband, and SNP treasurer Colin Beattie were arrested.

Sky News looks into what police are investigating, who has been arrested and what the reaction has been.

What are police looking into?

Operation Branchform is Police Scotland's investigation into possible fundraising fraud in the SNP.

Launched in 2021 following seven complaints to police, it is looking into whether £666,953 of money raised since 2017 specifically for a second independence referendum has been improperly spent on other activities.

Police are also looking into a loan of £107,620 made in June 2021 by Mr Murrell, while he was chief executive.

The loan was reported late to the Electoral Commission and two repayments were made later that year by the SNP, but there was £60,000 left outstanding.

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Who has been arrested?

Mr Murrell, 58, was arrested on 5 April and spent nearly 12 hours in custody as he was questioned by police before being released without charge but pending further investigation.

The Glasgow home he shares with his wife, who was SNP leader until March, was searched for two days, with a blue forensics tent set up in the front garden as officers removed large green boxes and were seen carrying spades into the back garden.

Detectives also seized a £100,000 motorhome owned by the SNP from outside the home of one of Mr Murrell's elderly relatives.

They searched the SNP's Edinburgh offices at the same time.

Ms Sturgeon unexpectedly quit as leader of the SNP after eight and a half years in mid-February, saying the job "takes its toll on you and all around you".

She later denied her husband's impending arrest was anything to do with her decision and said she had "no prior knowledge" he was to be arrested.

On 18 April, long-term treasurer of the party Colin Beattie, was arrested as part of the investigation.

He took up the role in 2004 and was briefly replaced by an internal election in 2020 by Douglas Chapman, who then quit in 2021, saying he "had not received the support or financial information required to carry out the fiduciary duties of national treasurer".

Mr Murrell took over as acting treasurer before Mr Beattie took up the role again in 2021.

What have senior SNP politicians said about the investigation and arrests?

On the loan from Mr Murrell, the SNP said at the time it was raised that it was a standard transaction to help with "cashflow" following the last Holyrood elections and they initially did not think they had to report it.

Humza Yousaf, who replaced Ms Sturgeon as SNP leader, said he was unaware the SNP owned the motorhome confiscated by police until he became party leader.

He also said he had not been aware the SNP's long-standing auditors, Johnston Carmichael, had quit in September 2022 and there is yet to be a replacement.

Asked whether it was wrong of Ms Sturgeon not to reveal to SNP leadership candidates the lack of auditors, Mr Yousaf responded: "Frankly it would have been helpful to know beforehand.

"I think people have a reasonable question to ask the party around transparency. I have committed to a review of governance with external input."

The SNP has said it cannot comment on the two arrests, with a spokesman saying: "Clearly it would not be appropriate to comment on any live police investigation but the SNP have been cooperating fully with this investigation and will continue to do so.

"At its meeting on Saturday, the governing body of the SNP, the NEC, agreed to a review of governance and transparency - that will be taken forward in the coming weeks."

Following her husband's arrest, Ms Sturgeon said: "The last few days have been obviously difficult, quite traumatic at times, but I understand that is part of a process."

She said she had not been questioned by the police but would "fully cooperate" if they do want to quiz her.