Patients at the emergency department in Swansea's Morriston Hospital were left sleeping in chairs for days, and without access to food and heating, according to a damning inspection.
The report, by Healthcare Inspectorate Wales (HIW) follows an unannounced inspection of the emergency department in September.
It identified 25 areas for improvement including patient privacy and dignity, nutrition and hydration, infection prevention and control, and staffing levels.
The report found an "older frail patient" had been lying, in pain, on an ambulance trolley for more than 11 hours.
"Patients who were in the department for excessive lengths of time were not offered food," inspectors noted.
When inspecting the waiting area, inspectors found one patient lying on the floor in a corridor. The patient said a relative had returned home to get them a pillow to help ease their discomfort.
One patient described spending a "stressful few days" in A&E where they were forced to sleep in a plastic chair, whilst being in pain.
Other patients claimed they had not been offered food and that they were still waiting for antibiotics more than five hours after doctors said they need them.
Staff complained the overcrowded emergency department was "not fit for purpose" with patients left without privacy or "heating facilities."
Figures published in October show 66.6% of patients are waiting less than 4 hours in Welsh NHS emergency departments before admission, transfer or discharge.
The Welsh Government's target is that 95% of patients should be seen within 4 hours.
In 2019, the average time patients spent in emergency departments was around 2 hours and 30 minutes.
This rose to a record high of 3 hours and 8 minutes in March 2022 but has since dropped by 15 minutes.
The inspection concluded that despite staff efforts, patients were not receiving quality care.
HIW noted the "intense pressure being experienced across Wales within primary care, ambulance services and emergency departments" but said there was a lack of timely care and treatment at the department.
Chief Executive of HIW, Alun Jones said: "There are mounting pressures on NHS services, and like all hospitals across Wales, Morriston Hospital continues to face extraordinary challenges due to increased demand and staff shortages.
"Patient flow is a nationally recognised problem, caused by system-wide pressures and HIW acknowledges the health board is working hard to make improvements and reduce waiting times. Our report sets out specific recommendations which the health board needs to address in order to improve.
"We will continue to engage with Swansea Bay University Health Board to ensure progress against our findings. Key staff at the health board have been positive in their response to our feedback and in our subsequent communication, with a clear commitment to addressing the issues highlighted."
Swansea Bay University Health Board welcomed the findings of the report and said it had opened a new acute hub at the hospital to treat patients.
A statement said: "Our senior clinicians and management team have already reviewed its findings in detail to produce a comprehensive action plan, and will work hard to implement the necessary changes in order to improve the service.
"As HIW points out, the inspection took place at a time of the intense pressure and unrelenting demand, which continues to be the case across the UK as we try to overcome the challenges experienced, brought about by the pandemic."
It added: "We are completely overhauling our acute medicine service from this week, with the launch of a new acute hub at Morriston Hospital. This is designed to take a considerable amount of pressure off the emergency department once it beds in over the coming months."