Crosshouse Hospital in Ayrshire had to reroute ambulances owing to excessive strain, according to a status report for Scottish hospitals published on Sunday
One patient had been holding out until 1 p.m. for 15 hours and 41 minutes.
A health board has apologised “unreservedly” after a patient was left in an ambulance outside an emergency department for more than 15 hours.
Joanne Edwards, director of acute services, said that despite staff working hard to see patients as quickly as possible, some waited “significantly longer than we would wish and we unreservedly apologise for that”.
NHS Ayrshire & Arran said it was experiencing “extremely high demand for our unscheduled care services”.
Just 61.9% of people in the week up to November 27 were dealt with inside that window – against a Scottish Government target of 95%.
At the Queen Elizabeth University Hospital, just 36.8% were seen and discharged within four hours, although the figure rose from 35.1% the week before.
Ms Edwards said: “Each patient attending our Emergency Departments is triaged on arrival and clinical teams prioritise them based on clinical need.
“NHS Ayrshire & Arran work closely with colleagues across all emergency services, including our colleagues in the Scottish Ambulance Service, to anticipate and mitigate against delays wherever possible.
“Ambulances are diverted during times of pressure across our sites and yesterday (Sunday, December 11) some ambulances were diverted from University Hospital Crosshouse to University Hospital Ayr.
“We continue to focus on discharges which supports the flow of patients from our emergency departments.
“We would like to thank patients for their help and understanding as we continue to work under extremely difficult circumstances.
“If we all work together we can ensure that our Emergency Departments are there to look after those who need them most.”