The BBC understands the national force asked the UK agency to carry out an independent review of its inquiry last year.

The terms and outcome of this exercise have not been made public.

The investigation has intensified since then with high profile arrests, searches and the seizure of a motorhome.

Police sources said it was "good practice" in cases of this nature for the inquiry team to ask another force to double check their work.

This is known as a "peer review".

According to one senior police source, a peer review is typically "conducted to check on the status, strategy and direction of an investigation".

They added: "The review checks that the lines of inquiry are correct, that nothing has been missed and that the rationale is proportionate and necessary".

The National Crime Agency is based in Westminster, London

The National Crime Agency specialises in the investigation of serious and organised crime across the UK.

It is not otherwise involved in the investigation into the SNP's finances and fundraising, known as Operation Branchform.

That began in July 2021 following complaints about how more than £600,000 of donations for a future independence referendum were used.

The SNP's former chief executive Peter Murrell and the party's ex-treasurer Colin Beattie were arrested, questioned as suspects and released without charge.

The former first minister Nicola Sturgeon has said she would fully cooperate with police if they wanted to speak to her.

Ms Sturgeon spoke to journalists in the Scottish Parliament last month

The Uddingston home she shares with Mr Murrell, who is her husband, was searched for two days last month. She later described the experience as "traumatic".

A further search was carried out at SNP headquarters in Edinburgh with officers removing boxes of material from the premises.

Police also removed a luxury motorhome from the driveway of Mr Murrell's mother's home in Fife.

Some in the SNP have publicly questioned Police Scotland's approach with the Glasgow MSP James Dornan describing it as a "fiasco" on social media.

Murray Foote, the party's former communications chief at Holyrood, last week said he was prepared to bet that no charges would be brought.

A Police Scotland spokesperson said: "As the investigation is ongoing we are unable to comment further".

New auditors

The SNP appointed new auditors this week after the firm Johnston Carmichael quit last September.

The party's Westminster group risks losing £1.2m in public funding if it cannot submit audited accounts by the end of this month.

The party as a whole could be fined if it cannot meet a separate deadline to submit accounts to the Electoral Commission in July.

On Wednesday SNP leader Humza Yousaf said he was optimistic the deadlines would be met but described the timetable as "challenging".