Hywel Dda University Health Board has apologised for forcing patients to wait in ambulances outside Bronglais Hospital in Aberystwyth.
Mark Brandreth, Director of Planning and Operations at the health board, said: "Any patient waiting in an ambulance is assessed, managed and treated according to their needs by our doctors and nurses."
Bosses say ambulance admissions across Wales are currently 22 percent higher than predictions for this time of year.
Some planned surgery has been postponed, to provide more resources to the emergency department.
The health board is asking people with minor injuries or illnesses to try other options, such as getting advice from a local pharmacist or GP, or using minor injury units, rather than going to Bronglais.