The Accounts Commission found council social work departments are facing “significant challenges” due to a range of financial pressures.
It estimated social work spending will need to increase by up to £667 million by 2020, a rise of 21 per cent, unless new ways of delivering services are brought in.
The local government spending watchdog concluded councils have coped well with the challenges they have faced in recent years but the country is now facing a “watershed”.
It warned the time has come for “frank discussions and hard choices” for social work services of the future.
The report, prepared by Audit Scotland, represents the first time the commission has looked at the entire range of social work services across the country.
It comes just days after social worker Lesley Bate, who was involved in the care of murdered Fife toddler Liam Fee, was struck off for misconduct.
Social workers in Edinburgh also came under fire over the murder of three-year-old Mikaeel Kular, whose mother beat him to death before dumping his body in a suitcase last year.
And Glasgow City Council’s social work services faced criticism after foster parent Dawn McKenzie was fatally stabbed by a 13-year-old boy in her care in 2011.
More than 200,000 people work in the sector, which supported and protected more than 300,000 vulnerable Scots in 2014/15.
The study found social work spending has risen by three per cent in real terms since 2011/12 and now accounts for nearly a third of overall council outlay. This came at the same time as council revenue funding fell by 11 per cent.
The analysis found local authorities have adopted various strategies to achieve savings, such as reducing services and cutting costs, but that there has been “little in the way of fundamental change in the way councils deliver services”.
Douglas Sinclair, chair of the Accounts Commission, said: “Increasing pressures on social work and rising expectations of what it should deliver can only intensify.
“Now is the time for some frank discussions and hard choices.”
Health secretary Shona Robison insisted the Scottish Government had always treated local councils fairly, despite budget cuts from Westminster.
She said:“One of our main focuses in the Programme for Government is the improvement of the child protection system and our commitment to have in place a skilled and competent social service workforce across all services, which can deliver high-quality, personalised, safe, continually improving services for people and communities in Scotland.”
But Scottish Labour communities spokesman Alex Rowle said the report shows “the human cost” of SNP cuts to councils.