Patients at Ysbyty Glan Clwyd's Emergency Department in Rhyl are 'at risk' according to a new report published on Monday.
The report, carried out by Health Inspectorate Wales (HIW), says there is urgent improvement needed and that adequate arrangements are not in place to support the delivery of safe healthcare.
Children in the hospital's paediatric waiting area of the emergency department were said to be at 'serious risk of harm'.
HIW completed the unannounced inspection of the emergency department on three consecutive days in May and was undertaken as a follow up on significant concerns HIW identified during a previous quality check undertaken in March.
Inspectors concluded that there was a lack of improvement to an acceptable standard and as a result, HIW designated the emergency department in Ysbyty Glan Clwyd as a Service Requiring Significant Improvement (SRSI)
The inspection identified several additional areas of concern relating to patient safety.
Chief executive of HIW, Alun Jones said: "The findings within this report are extremely concerning, and we have urged the health board to take immediate action to protect patients from the risks identified.
"The designation has been made to strengthen and accelerate the measures taken to drive timely improvements within the service.
"We will be working with the health board to ensure robust improvements are made in a timely manner, and will consider the timing of any follow up activity, which will need to be evidenced."
The health board boss has apologised and says clinical staff have been under immense pressure and they are committed to recruiting more employees.
What the report found:
Patients were not monitored sufficiently, including a suspected stroke patient and a patient considered at risk of taking their own life.
Used underwear among dirty equipment found in cupboards.
Doctors were left to 'come across' high-risk patients instead of being alerted to them.
Prescription only medicines and equipment such as needles found in cupboards left unlocked and unsecured.
Children at serious risk of harm as members of the public could enter paediatric areas unchallenged and without authorisation.
Evidence of children leaving hospital without having been seen by a doctor or being discharged against medical advice
Many staff said they were unhappy and struggling to cope, unsupported by senior managers.
Families of patients who have died at the hospital say improvements must be made urgently to avoid other patients and family's experiencing the same.
In February, Michael Hugh Matthews from Rhyl died from a heart attack at the hospital after a four hour wait at the hospital's A&E department.
A statement from his family read: "We are bitterly disappointed that over 2 years since we lost Mike, this report clearly shows that lessons have still to be learnt and patients are still being put at risk.
"BCUHB needs to put in place urgent resources to ensure that patient care and safety is of utmost importance.
"We don't want another family to go through the devastating loss of a loved one in the most tragic of circumstances. Mike was an amazing dad, son, brother, uncle and friend to so many and we all miss him everyday.
"We are anxious for the inquest to take place, so we can finally learn and understand the full circumstances of his loss."
Jo Whitehead, chief executive of Betsi Cadwaladr University Health Board said: This latest report shows some patients have received a standard of care well below what they, and we, expect.
"I apologise unreservedly to those patients who didn't receive the care they deserve.
"It is most disappointing to see areas highlighted by the remote quality check had not all been addressed by the time of the unannounced inspection, some two months later.
"I have made several visits to speak with our hard working staff in the department, to discuss how we can better support them to do their jobs more effectively, as we all have a shared responsibility to get this right.
"Clinical staff have been under immense pressure and often understaffed. Our improvement approach seeks to enable staff to be open about the pressures they are under and the things preventing them from giving the care we all want to provide and for us all to provide the support required to put things right.
"Staffing continues to be an issue but we are committed to recruiting the permanent staff we need to ensure patients get the best possible emergency care, which is what happens in the vast majority of cases and of course, I sincerely thank staff for their continued hard work."
A spokesperson for the Welsh Government spokesman said the standard of care "falls considerably short" of what it expect for patients.
It said it took immediate action in June by escalating this service to Targeted Intervention status said it is assured by the health board that many of these actions have now been implemented.
It reaffirmed that and it believes it is safe for people to continue attending the emergency department at Ysbyty Glan Clwyd.
However, opposition parties have accused ministers of 'continuing to fail' patients in the north of Wales.
The Welsh Conservatives' shadow minister for North Wales, Darren Millar, said: "Once again, the Labour Government in Cardiff Bay is failing the people of North Wales.
"It will be deeply alarming to people across the region that the situation at Ysbyty Glan Clwyd still isn't getting things right despite being put into targeted intervention – Labour ministers need to step up and take responsibility for their failings.
"It is becoming ever more evident that Betsi Cadwaladr Health Board was only taken out of special measures ahead of the Senedd Elections for purely political reasons.
"It is unbelievable that Labour are focusing their efforts on expanding the Senedd rather than address the issues facing the NHS, especially in North Wales."
Plaid Cymru's spokesperson for health and social care, Rhun ap Iorwerth, said: "Another week, another damning report, and more concerns for patients safety. When will this end?
"This report again speaks of patients being put at risk and staff ‘working above and beyond in challenging conditions’.
"I have little faith in Welsh Government’s ‘targeted interventions’, and once again I call on the Health Minister to look at how starting again with new health structures in the north could provide the fresh start patients and staff need.
"It’s not because I particularly want a health reorganisation but that I think we have little choice."