Adults who fund their own care are now able to make an official complaint for the first time.
The Local Government Ombudsman (LGO) has launched a service similar to that available to people whose care is arranged and funded by local authorities - which the LGO has dealt with for more than 35 years.
The Health Act 2009 amended the Local Government Act 1974 to give the LGO service the new powers.
Local Government Ombudsman, Tony Redmond, said: “Until now, the only form of redress for people in privately-funded care was through the care provider’s own complaints procedure or going to court.
“From today, if service users, a member of their family or others affected by the service have suffered an injustice, we may be able to help.”
Mr Redmond added that, in most cases, the LGO would only consider a complaint once the care provider had had “a fair opportunity to put the situation right”.
Care Services minister Paul Burstow said: “Everyone should be guaranteed good quality care and dignity however their care is funded.”
The new powers will allow the LGO to investigate complaints about services that are registered under the new Care Quality Commission essential standards that also come into force on 1 October.
The types of complaints the LGO is expecting to deal with cover a variety of services such as needs assessments, poor care quality and fees and charges from care homes, personal care at home and supported living services.