Mr McRae's death in 1985 has gone down as suicide, although many believe he was murdered. It has also been officially denied that he was under surveillance.
But Iain Fraser, who worked as a private investigator after leaving the police, has revealed he was asked by a mystery client to spy on Mr McRae just three weeks before he died. He has added his voice to calls for a public inquiry into the death.
Mr McRae, a lawyer and anti-nuclear campaigner, was found dead in his crashed car on the A87 near Kintail in Wester Ross. At first it seemed he had died in the accident, but a gunshot wound was later found.
The gun was recovered some distance from the car, but there were no fingerprints. No fatal accident inquiry was held.
Mr Fraser, 65, who runs a hotel in Cullen, said he was paid 135 by the caller to watch Mr McRae on a Saturday in 1985. He said: "I had no idea who the client was, but in the murky world of private investigation that was not unusual.
"The cheque came from Newcastle. I can't remember the individual who signed it, I wished I had kept a copy now, but I had no idea at that time this was going to rear its head again."
Mr Fraser was based in the same building as Mr McRae's office in Glasgow. "I was surprised to be asked to do the job as there was a possibility that, having met each other, he may recognise me."
Mr Fraser followed Mr McRae from mid-afternoon until about 6pm and from 8pm to nearly 11pm. During that time the solicitor left the SNP headquarters in Edinburgh to meet two people in Morningside and attended a concert in the Queen's Hall.
Mr Fraser said: "My client was able to tell me exactly what Willie's movements would have been that day. I just thought it was a normal, domestic job and had no idea it could have such serious implications."
Mr Fraser has spoken up now after meeting another former policeman, Donald Morrison, who spoke to Mr McRae just before he left on his fateful trip to Kintail and is convinced his death was not suicide.
Mr Fraser added: "There seems to be so much smoke and mirrors involved in this case. If this sparks off an inquiry, I would be happy because there are a number of things that remain unanswered.
"There are inconsistencies at the crime scene and it seems there was a closing of ranks of the establishment."
Repeated requests for an official inquiry into Mr MacRae's death have been turned down. Last year, Elish Angiolini, the Solicitor General, refused a request from Fergus Ewing, the SNP MSP, to discuss allegations that Mr MacRae was under surveillance in the weeks leading up to his death. She also said a further investigation was not justified.
Mr Ewing said he plans to meet Mr Fraser and added: "There remains a cloud of suspicion over Willie McRae's death and this is another emerging piece of possible evidence."
The Crown Office declined to comment yesterday.