A final report on the police investigation into SNP finances will "probably" be passed to prosecutors, a former top cop has predicted.

Sir Iain Livingstone, who retired this month as Chief Constable of Police Scotland, also said in his first interview to be published since his retirement, choosing not to investigate complaints against the SNP would have been a "grotesque neglect of my duty".

It is his latest defence of the probe which comes in the wake of attacks on it by some in the SNP, including its current chief executive Murray Foote.

READ MORE: Operation Branchform: SNP urged to give new assurance

Mr Foote, who took up his new post yesterday, launched a ferocious attack on Operation Branchform in May, calling the search by officers of Nicola Sturgeon's and Peter Murrell's house a "grotesque circus".

He also accused officers of “storming” the SNP headquarters and wrote that the probe could turn into a debacle with "serious consequences" for the authorities like the failed Rangers prosecutions which cost taxpayers £50million. 

Mr Foote, who replaced Mr Murrell as the SNP chief executive, also said in his criticism of the probe in May that he would be willing to gamble that the criminal investigation would result in no charges being pressed and speculated that it was a “wild goose chase”.

Separately, James Dornan, the SNP MSP, accused the media and police of “some kind of collusion” over the arrests of Ms Sturgeon, Mr Murrell and Colin Beattie, the former SNP treasurer. Mr Dornan described the image as akin to “Fred West’s house when they come to look for a paper trail”.

Speaking to Holyrood magazine Sir Iain again defended the decision to erect a tent outside Ms Sturgeon's and Mr Murrell's house while officers searched inside.

Sir Iain also said he did not know how the investigation would be resolved - but predicted it would be left up to prosecutors on whether to press charges.

"If I hadn’t been pursuing Operation Branchform, if we didn’t have the detectives investigating the matters that we are now looking at, it would be a grotesque neglect of my duty, not only legally, but morally," he said.

READ MORE: Operation Branchform: Police chief rejects attacks on SNP probe

"I don’t know how the matter will ultimately be resolved, that will probably be outwith the hands of the police service, but it will go into the independent prosecution service and if need be, it will go into the hands of the court system.

“But the police responsibility is to investigate thoroughly, to look for evidence and if there is sufficiency of evidence that suggests criminality, we would then report that matter to the Crown [Scotland’s prosecution service].”

He said that each step has been taken with a “sense of necessity and proportionality”, adding: “Uninformed speculation, assertions that I know to be factually inaccurate, a level of language used about the action that we’ve taken that is not based on the facts, is wrong.”

The former chief constable added that public commentary about the probe was "very damaging to the principle of the “rule of law” and "potentially damaging" to individuals involved.

In a vigorous defence of Operation Branchform, which has expanded from its original remit of investigating the use of £600,000 raised by the party to campaign for independence, Sir Iain said “when opinion gets polarised . . . it doesn’t assist the administration of justice”.

He added: "I know the rigour that has gone into this investigation, because, you know, I've kept a very close eye on it since the outset."

Operation Branchform was launched two years ago to examine how donations to the SNP to fund a second independence referendum campaign were being spent. It saw dozens of officers search a home belonging to Ms Sturgeon and her husband, Mr Murrell, the former SNP chief executive for two days in April.

Sir Iain said: "The tent was there, as were all the other measures, to protect the interests of justice and to protect the individuals involved, so it was a proportionate and necessary step".

Mr Murrell and Ms Sturgeon were separately interviewed by detectives for several hours in connection with Operation Branchform before being released without charge, pending further investigations.

Mr Beattie, the party's former treasurer, was also arrested and released without charge pending further inquiries.

Ms Sturgeon has repeatedly denied any wrongdoing.

Mr Foote quit as the SNP’s Holyrood communications chief in March after being misled by colleagues at HQ about the party losing a third of its members.

After unwittingly misleading the media in turn, he resigned saying the episode had created “a serious impediment to my role”.

His exit piled huge pressure on then chief executive Mr Murrell, who resigned the next day after two decades in charge.