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.
More top news
It's not too bad out there tonight. A few showers over northern areas, but dry elsewhere with temperatures degrees milder than last night.
Traders across Wales have told ITV News the migrant crisis in Calais is having a 'devastating' impact on their business.
The Balmoral is back but for how long? Volunteers appeal for public support to keep her sailing the Bristol Channel.