Half of A&Es in England are not good enough - and there are no guarantees services will get better despite the extra money going into the NHS, the regulator says.
The Care Quality Commission said people were turning to A&E in a crisis because of a lack of support in the community.
It said services would remain under pressure until a funding solution was found for social care.
The government pledged in the Queen's Speech to reform social care.
It has set out a five-year funding plan for the NHS, which will provide the health service with an extra £20bn a year by 2023.
But attempts to reform the social-care system - run by councils and heavily means-tested - have been delayed.
It is estimated around 1.4 million older people do not have access to all the care they need because of inadequate access to services such as help in the home and care homes.
CQC chief executive Ian Trenholm said this was having a knock-on effect on hospitals.
Some 44% of A&Es are now rated as requiring improvement and 8% are deemed inadequate - a drop in performance from 2018 - the regulator's annual report showed.
Mr Trenholm said A&E was "the department that we are most concerned about", adding that there was a "rising demand and people struggling to provide high-quality care".
It is more than four years since the four-hour A&E waiting-time target has been achieved.
Mr Trenholm said until a social-care settlement was reached - which the government has promised is imminent - he was not confident services would improve.
Dr Nick Scriven, of the Society for Acute Medicine, said the system was being "failed" by those in power.
"As we move into the autumn, pressure has remained relentless," he added.
The CQC also expressed strong concerns about the state of services for people with learning disabilities and mental health.
Some 10% of inpatient services for people with learning disabilities and autism were rated inadequate - up from 1% last year. Child and adolescent mental-health services and psychiatric units for adults had also deteriorated.
Inspectors said too many were being looked after by staff who lacked the skills, training and experience to support people with complex needs.
The warning comes after the BBC Panorama programme exposed abuse, neglect and mistreatment of adults with learning disabilities at Whorlton Hall specialist hospital.
The CQC said it had seen a rise in complaints since the programme aired in May.
But the report contained plenty of positives, too - 95% of GP practices were rated outstanding or good and core services other than A&E showed signs of improvement.
Even in social care 84% of services were rated good or outstanding - it was just that there are not enough to go around, leaving some people without the requisite care.
The CQC said the number of care-home places was declining - and there was not enough home care to keep up with demand.
The government made a commitment to reform social care in the Queen's Speech on Monday.