Long handover waits for patients stuck in ambulances outside hospitals increased the risks to them, inspectors have found.
West Midlands Ambulance Service (WMAS) failed to meet response times and handover targets during an inspection in November, watchdog the Care Quality Commission (CQC) said.
But crews were praised for their care and compassion to patients in their vehicles during the lengthy delays.
WMAS said handover times had improved.
The NHS target is for patient handovers to be completed in 15 minutes, with none waiting more than 30 minutes.
But on the day of the inspection, one patient waited more than 19 hours in an ambulance to be admitted, and the average wait was about 6.5 hours.
“The delays in handing people over at hospitals meant that ambulances and crews couldn’t be made available to attend other calls,” Sonia Brooks, from the CQC, said.
“These lengthy delays at hospitals increased risk to people, particularly those that had been lying on trolleys or stretchers in ambulances for long periods of time.”
The CQC spoke to dozens of paramedics plus 15 patients and said staff treated people with compassion and kindness, and respected their privacy and dignity.
Paramedics were also praised for their infection prevention and control and also their communication with each other.
But alternatives to taking people to hospital were not always known by staff and some senior leaders were not available at busy times which left workers disheartened, the CQC said.
WMAS has been urged to work with other organisations to cut waiting times at hospitals and also to make sure patients waiting in ambulances receive personal care.
The trust remains rated as outstanding overall, the CQC says.
WMAS said it was pleased inspectors saw the lengths to which staff went in their care for patients.
It said handover delays had improved significantly since the start of the year across the region.
A spokesperson said: “We will continue to work with the hospitals to find new ways of reducing the time patients are left on ambulances so our crews can respond more quickly to patients in the community and save more lives.”