The Care Quality Commission has rated Orchard Lodge care home as 'inadequate' following an inspection in July.
CQC inspectors found the home, in Dorking Road, Warnham, was 'inadequate' for being safe and well-led, 'requires improvement' for being caring and effective and 'good' for being responsive to people’s needs.
Orchard Lodge provides accommodation in three units, which are all on one site, providing nursing and personal care for up to 33 people who may have learning disabilities, physical disabilities and sensory impairments.
Most people living in the service had complex mobility and communication needs.
A CQC spokesman said this inspection was bought forward to July because, following the identification of risks relating to people's care, the service had been subject to a period of increased monitoring and support by commissioners.
He added that the service has been the subject of eight safeguarding investigations by West Sussex County Council and partner agencies. As a result of concerns raised, the provider is currently subject to a police investigation.
Debbie Ivanova, deputy chief inspector of adult social care, said she was "concerned" that insufficient staffing levels were frequently raised during the inspection.
She added: "By not having enough staff in a home it can impact quite severely on the quality of care and brings into question how safe the service can be.
"We also received a number of safeguarding concerns about the management of complex health conditions (epilepsy, asthma and dysphagia - difficulty swallowing - the care of percutaneous endoscopic gastrostomy - PEG - feeding tubes for people who were not able to take food and drink by mouth and the deployment of suitably qualified and skilled staff around the service in general.
"We have found gaps in training provided to staff.
"Some people living in Orchard Lodge had a learning disability but not all staff had received specific and relevant training. A significant amount of people lived with epilepsy however, some staff had never completed an epilepsy training course.
"We have rated Orchard Lodge overall as 'inadequate' and placed the service in special measures.
"We are working closely with our partner agencies and will continue to monitor this service. We will inspect again within six months to make sure improvements have been made.
"I expect that the provider, SHC Clemsfold Group Limited will do what is required for the sake of the people in its care, but if we find that the service remains inadequate, we will not hesitate to take further action."
A spokesman for Sussex Health Care said: "We are deeply disappointed by this report, which we believe is significantly flawed and apologise to residents and relatives for any distress it may have caused.
"This is the first time in over 30 years of providing care and support that we have ever received a rating of this kind.
"Whilst we take feedback from the CQC very seriously, and have taken steps to address specific concerns, we believe that the overall rating is neither fair nor proportionate, particularly given that the home was rated as 'good' in all five key categories at its previous inspection in November 2016.
“The report agrees that staff are kind and caring, and that residents are happy with the care and support they receive. Throughout the report, it is acknowledged that feedback from relatives is also consistently positive, including relatives who have said that ‘the care is second to none’, and ‘In seven years of my [family member] being at Orchard Lodge there has never been cause to complain.’
"Yet this positive feedback from those who visit the home to see their loved ones on a daily basis is not reflected in the report’s findings.
“Nevertheless, we fully accept that inspectors raised a small number of concerns.
"Where specific areas for improvement have been identified, we have taken steps to address each point. This includes providing additional management support for the home’s new manager, appointing an independent safeguarding expert to provide direct support, recruiting additional staff and implementing further training.
"The home has subsequently had a monitoring visit by the county council, from which verbal feedback has been positive.
"We expect written feedback confirming this soon."
“The health, wellbeing and safety of the people we support is our top priority and we are absolutely committed to providing the high quality care residents expect and deserve. We look forward to the CQC returning within six months to see the progress that has been and continues to be made.”