More than 100 home-care services for the elderly have been given low ratings for how they are run, according to latest figures from the Care Inspectorate.
Another 80 care-at-home providers - as many as one in 10 - have been rated adequate or below for care and support following inspections this year, and 70 have been given low ratings for the quality of staffing.
In a separate report, five per cent of private companies were graded "unsatisfactory or weak for every theme".
The report says: "There was a notable increase in the number of poorly performing services during 2012-13", adding: "Services achieving these very concerning grades generally improve, but these improvements are not always sustained."
Annette Bruton, chief executive of the Care Inspectorate, said the failures should not be tolerated. She said: "We should have a zero-tolerance approach to the quality of care that the most vulnerable people get in our society.
"We should not tolerate that five per cent failure of service."
More than 62,000 people receive some form of care in their own home in Scotland and there are 814 different registered providers of services, including private firms, councils, the NHS and not-for-profit organisations.
The Care Inspectorate assesses and grades them all annually and investigates complaints. Its first report on home care paints a damning picture of services that were given poor ratings between 2010 and 2013. It also describes risk assessments not being in place for pensioners, or being carried out by staff who were not qualified.
In some cases, there were "staffing levels which were clearly insufficient to meet the needs of the individuals" and staff were not trained or qualified to carry out the tasks required. Practices around the way medicines were administered were sometimes found to be unsafe.
There has been little change in the proportion of services given poor grades since March last year. The latest figures for March this year show 10.9 per cent of providers rated adequate or below for the quality of care and support, 9.5 per cent for quality of staffing and 15 per cent for management and leadership. This compares to 11 per cent, 10.9 per cent and 14.3 per cent last year
Kenny Campbell, a complaints team manager for the Care Inspectorate, said providers did respond to the Care Inspectorate's interventions. "The most common complaints that we see coming through the door are mostly about people not turning up, not turning up on time, not knowing who is going to turn up or the number of different people turning up to give care," he said.
Ranald Mair, chief executive of Scottish Care, which represents independent care providers, said it should be acknowledged that more than 80 per cent of home care services were rated good, very good or excellent.
He said that local authorities commission independent companies to deliver home care so if pensioners only receive 15-minute visits it was due to that contract, "as opposed to what the provider would want to deliver". He also said local authorities funded their own home-care services at a cost of £23 an hour, while independent providers were paid an average of £13.50.
A Scottish Government spokesman said there were "still too many services that are not providing the level and quality of care that the Scottish Government, and people using services, expect. This is why we have a rigorous inspection regime in place to highlight any areas of concern."