Solicitor General Lesley Thomson expressed frustration at being unable to prosecute rape suspect Lee Cyrus, and said one of the reasons was the rule that evidence against an accused must be corroborated by another source.
But a leading lawyer last night responded by saying changes to the law need more careful consideration and could lead to people being unfairly convicted.
A 23-year-old woman was raped in her home in Perth and pensioner Marjory Moran was attacked on her doorstep. in the city in 2012 while Lee Cyrus, 48, was on the run. He had fled from an open prison in England where he had been convicted of sex offences.
He was the only suspect in the Perth attacks and was charged with both, but there was outrage last month when it was revealed he would face no action in Scotland.
Proposals to abolish corroboration are among reforms in the Scottish Government's Criminal Justice Bill currently going through the Scottish Parliament.
Ministers believe the change would mean more rape and sexual assault cases would get to court, although Scotland's most senior judges and lawyers have warned the move could lead to miscarriages of justice.
And last night Ian Cruickshank, convener of the Law Society of Scotland Criminal Law Committee, said law reform should not be based on isolated cases.
"It's important that any reforms to our criminal justice system are well-considered, workable and sustainable and we don't believe abolishing the requirement for corroboration without further detailed consideration given to all other safeguards will make for good law," he said.
"It is a very complex issue and we recognise that different views have been put forwards by other individuals and organisations who consider that the current requirement for corroboration can prevent cases from being prosecuted and that in some circumstances people alleged to have committed crimes, particularly sexual offences, are beyond the reach of the criminal law.
"However, we continue to have very serious concerns that the current proposals to remove the requirement for corroboration without considering other safeguards will increase the risk of miscarriages of justice … This is why we are calling for a review of the law of corroboration to examine whether it should be retained in full, retained in part as in other jurisdictions, or abolished."
In a letter to SNP MSP Annabelle Ewing, the Solicitor General defended the handling of the Cyrus case but said: "I can understand the disappointment and concern expressed about this case. It is extremely frustrating to prosecutors that no action could be taken.
"Indeed, it is extremely unsatisfactory for victims to be denied access to justice because, despite their account of what has happened, a lack of corroborative evidence means proceedings cannot be raised …
"As you are no doubt aware, the Crown Office and Procurator Fiscal Service fully supports the proposals of the Scottish Government to abolish the requirement of corroboration, and believes that by doing so, more victims of crime will have access to justice."
Ms Thomson said Crown lawyers met both victims to explain the decision, and reserved the right to try Cyrus if new evidence emerged.
Ms Ewing said the case shows change is needed: "I can fully understand the frustration of prosecutors … But much more importantly, victims of serious offences are being denied justice."