Plans for a private prosecution of Glasgow bin lorry crash driver Harry Clarke have not been granted approval by the Crown Office.
Lawyers for the family of victims Jack and Lorraine Sweeney and their granddaughter, Erin McQuade, delivered a Bill for Criminal Letters to the Crown Office last week seeking "concurrence" for the rare move.
Lord Advocate Frank Mulholland has declined to support the prosecution but it can still continue and is now likely to be ruled on by High Court judges without his approval.
A Crown Office spokesman said: "The Lord Advocate has declined to grant concurrence to the Bill for Criminal Letters seeking a private prosecution of the driver of the bin lorry involved in the George Square fatalities on December 22 2014.
"The original decision not to take criminal proceedings was made on the basis that there was insufficient evidence in law to prove that a crime had been committed and that position remains unchanged."
Six people died when the vehicle went out of control in Queen Street in December 2014.
A Fatal Accident Inquiry (FAI) held last year heard that Mr Clarke lost consciousness at the wheel and that he had a history of health issues - including a previous blackout in 2010 when at the wheel of a stationary bus - but had not disclosed his medical background to his employers or the DVLA.
The Crown Office has consistently said there is insufficient evidence in law to raise criminal proceedings against Mr Clarke.
Another bid for a private prosecution submitted by the families of two young students knocked down and killed by a driver with a history of blackouts in 2010 has also been rejected by the Crown Office.
Mhairi Convy, 18, and Laura Stewart, 20, were walking in Glasgow's North Hanover Street on December 17 2010 when a Range Rover apparently lost control, mounted the kerb and hit them.
William Payne, who was 50 at the time of the crash, appeared at Glasgow Sheriff Court in November 2012 accused of causing death by driving while uninsured but charges against him were dropped the following November.
A Fatal Accident Inquiry (FAI) was held in 2014 into the deaths of the two women and Sheriff Andrew Normand found that five ''reasonable precautions'' could have prevented the accident, which happened after Mr Payne suffered a ''vasovagal episode'' and temporarily lost consciousness, losing control of the vehicle.
The families of Ms Convy and Ms Stewart have consistently called for Mr Payne to face a criminal trial and a Bill for Criminal Letters was sent to the Lord Advocate last week in the wake of the move by the families of the bin lorry crash victims.
Relatives of Ms Convy and Ms Stewart attended part of the bin lorry crash FAI last year in support of the victims' families.
A Crown Office spokesman said: "The Lord Advocate has declined to grant concurrence to the Bill for Criminal Letters seeking a private prosecution of the driver of vehicle involved in the North Hanover Street fatalities on December 17 2010.
"The original decision not to take criminal proceedings was made on the basis that there was insufficient evidence in law to prove that a crime had been committed and that position remains unchanged."
Permission may now be sought from High Court judges.