Scotland's top prosecutor has strongly defended the Crown's decision not to prosecute the driver in the Glasgow bin lorry tragedy, saying he has "no doubt that the decision was the correct one in law".

Lord Advocate Frank Mulholland QC said he understands the anger and upset of the families who lost loved ones and accepts the decision not to bring criminal charges against Harry Clarke was "unpopular".

But Mr Mulholland insists the Crown's senior lawyers were unanimous in the view that there was insufficient evidence in law to raise criminal proceedings against Mr Clarke on charges including causing death by dangerous driving.

He said the view was reached in full knowledge of all the major facts which emerged during the fatal accident inquiry (FAI) into the crash and rejected suggestions that decision-making in the case was rushed, arguing that past inquiries into some of Scotland's most high-profile tragedies were raised within a similar timescale.

Setting out the reasons behind the decision in detail, the Lord Advocate said: "I can well understand and appreciate how upset, how angry, families of those who lost their loved ones, those injured, would feel (given that) no-one has been held criminally responsible, and I fully accept that this is an unpopular decision.

"But you can't take decisions on the basis of whether or not they are popular. They have got to be taken on a hard, cold analysis of the evidence and that's what happened in this case."

The Crown has been criticised over its decision not to prosecute Mr Clarke, 58, who was unconscious at the wheel of the truck when it careered out of control in Glasgow city centre on December 22, killing six people.

The prospect of a private prosecution against the driver - who had a history of health problems which he failed to disclose and a blackout behind the wheel of a stationary bus in April 2010 - has been raised by some of the relatives of those who died.

Mr Mulholland said that for a criminal case to be brought by the Crown, it must meet two legal tests, beginning with establishing whether there is sufficient evidence in law. If that test is met, the Crown moves then to assess whether it is in the public interest to prosecute.

"In this case, if we got to the public interest it would be an easy decision - clearly in the public interest to prosecute - but that's predicated on there being sufficient evidence in law, which there is not," said the Lord Advocate.

"If there is insufficient evidence in law and proceedings are taken by the Crown, then that's an abuse of process, hugely damaging to the Crown. We cannot do that, it's an affront to the rule of law."

Mr Mulholland has now set out a number of reasons behind the decision not to charge Mr Clarke with causing death by dangerous driving.

Prosecutors would have to prove that Mr Clarke was in control of the vehicle at the time of the incident but, because he was unconscious, he was not in control, so "the necessary criminal intention" is not present, the Lord Advocate told Press Association Scotland.

Furthermore, he argues, there is insufficient evidence to establish "foreseeability" and prove that Mr Clarke knew he was unsafe to drive that day.

Mr Mulholland said: "When you put all the evidence together, it's quite clear and I have no doubt that you cannot establish, cannot prove, that MrClarke knew that he was unfit to drive that day as a result of an ongoing medical condition. If you can't prove that, then you cannot prove criminal intention."

Similar considerations applied to other allegations, such as obtaining the council job by fraud.

Mr Mulholland said: "The same evidence in general terms applies - namely he's given a reference by First Bus to Glasgow City Council, he's not told he's unfit, he's not told he needs to notify DVLA, there's no evidence that he must have known he was unfit to drive. So, you cannot prove the necessary criminal intention for that common law fraud."

The Lord Advocate said the Crown's top lawyers were united in their view that Mr Clarke could not be prosecuted.

He added: "The unanimous view of prosecutors, the Crown Office lawyers and Crown Counsel who took the decision was that there was insufficient evidence in law.

"As Lord Advocate, when I became aware of the concern that there may be about whether this decision was correct, it would be remiss of me not to check it, so I did check it.

"I checked the papers, any evidence upon which the decision was based, I spoke to people involved in the investigation and inquiry, and I have no doubt that the decision was the correct one in law."

The Lord Advocate also refuted claims that the Crown had been hasty in reaching its decision not to prosecute, paving the way for the FAI to take place.

There were no "barriers" to taking a decision, he said, unlike in the case of the Clutha helicopter crash, where a report from the Air Accidents Investigation Branch is awaited.

He also said the FAI into the Ibrox disaster which claimed 66 lives in 1971 began around six weeks after the tragedy. The inquiry into the 1996 Dunblane massacre commenced around two and a half months after the tragedy without any suggestion that it had been rushed.

"The Crown doesn't operate in a vacuum. We know that there's a high level of interest and concern, families need answers. We're well aware of that and that's why we put together a team to deal with this as quickly as possible without compromising thoroughness."

As for whether law changes are required in light of the bin lorry tragedy, that is a matter for the FAI, not the Lord Advocate.

Mr Mulholland said: "It's quite clear that there's a number of issues that have been raised by the court and we await Sheriff Beckett's determination with interest."

Erin McQuade and Jack and Lorraine Sweeney, from Dumbarton, West Dunbartonshire, Stephenie Tait, 29, and Jacqueline Morton, 51, both from Glasgow, and Ms Ewing, 52, from Edinburgh, were killed as the lorry driven by Mr Clarke veered out of control during a routine rubbish collection.