The ashes of 40 youngsters who died almost three decades ago may have been scattered in a garden of remembrance without their parents' consent, a report has revealed.

Auditors said they had been unable to confirm if parents of children cremated at Aberdeen's Hazelhead Crematorium over the period 1984-85 had agreed to this because the paperwork was no longer in existence.

The PricewaterhouseCoopers (PWC) report, prepared for Aberdeen City Council, explained cremation forms are only kept for 15 years – in line with directions from the Institute of Burials and Cremation Authorities.

PWC examined the practices and procedures carried out by staff at Hazelhead in the light of the Mortonhall scandal in Edinburgh – where it emerged staff at the council-run crematorium in the capital had secretly buried babies' remains without the knowledge of their families.

The PWC report also revealed that from April 1, 2007 to December 31, 2012 there were 40 cases where no ashes were retrieved when infants aged under two had been cremated.

Aberdeen City Council has previously said that the "intense heat and the turbulent flow of gases during the cremation process" means there are "no recoverable remains from a baby and infant up to the age of approximately 18 months".

The report said that while bereaved parents are informed there will not be any remains when a baby up to the age of 18 months is cremated, "there is not a formal documented policy or formal correspondence that is issued to bereaved parents setting this out".