David Gilroy, 52, was jailed for life in 2012 for murdering his former colleague Suzanne Pilley.
Gilroy’s sentencing was the first time TV cameras were allowed in a Scottish court but he has continued to protest his innocence ever since.
Miss Pilley’s body has never been found and there was no DNA or forensic evidence in the car or elsewhere linking Gilroy to the crime.
It has never been definitively proven that the 38-year-old bookkeeper died at all, but key evidence at the trial related to the silver Vauxhall Vectra.
The prosecution alleged Gilroy had made an unnecessary trip to the West Highlands, by an unusual route, and damage to the car suggested he had driven it off the road.
A gap of several hours between his departure and arrival, along with other circumstantial evidence, helped convince the jury he had detoured to dispose of Miss Pilley’s body in the wilderness somewhere in Argyll.
Gilroy’s appeal was rejected by the Court of Criminal Appeal in Edinburgh in December 2012.
Police Scotland has written to Gilroy in Perth Prison to tell him that if he does not wish to collect the Vectra, it will be disposed of by October 3.
Proceeds from any sale of the car will go to Police Scotland funds.
But Gilroy has written to the company currently holding the car - 911 Recovery - arguing Crown Office rules state all evidence used in the trial must be retained safely for the duration of his sentence.
His family believes the car could be crucial in their ongoing battle to prove his innocence.
In a letter to the Chief Constable of Police Scotland Philip Gormley, who is on special leave while facing allegations of bullying and misconduct, Gilroy claimed his appeals were ongoing and he plans a further application to the SCCRC.
He wrote: “I am extremely concerned that this key item of evidence... is being considered for disposal in the manner described...
“The case of which I have been convicted HMA v Gilroy was one in which there was no body, no forensic evidence and no witness to the crime.
“I continue to state my innocence and will be continuing to take steps towards establishing this.”
The car has already racked up more than £1,000 in storage fees, which Gilroy will have to pay to retrieve it, but doing so is likely to render it unusable as evidence.
In a letter to the Lord Advocate, Gilroy’s stepmother, former MP Linda Gilroy, states: “Even if he were to collect the car and find a means to pay the fees demanded by 911 Recovery, David is advised that the Crown has refused in other similar circumstances to accept a defence case because the production [item of evidence] had not been in the continuous custody of the Crown/police.”
She added: “We are shocked the Crown are not prepared to keep evidence which was central to their case that David covered up the murder he was supposed to have committed.
“To dispose of the evidence at this stage could deny David vital opportunities to add to the uncertainty that has already cast on the Crown case.”
The Crown Office and Procurator Fiscal Service (COPFS) declined to comment on Gilroy’s complaints.
A spokeswoman confirmed the release of the car had been triggered by COPFS, as a result of the conclusion of the appeal and Gilroy’s failed SCCRC application.
They are not concerned about what happens to an item of evidence, held by the police once released.