Hospital emergency departments in Wales are "on the edge", with some patients waiting for more than 24 hours, according to a leading doctor.
There are problems with recruiting emergency medicine consultants and issues with bed blocking, Dr Robin Roop, of the Royal College of Emergency Medicine, warned.
Dr Roop, the head of the college in Wales, said A&E departments were at "critical levels" and the problem had persisted for a year rather than being a winter crisis.
"The health boards have learnt from previous years about winter pressures and what needs to be done - things like expanding bed capacity and changing elective operations - they've worked to a small degree," he told the BBC.
"But we still see a reduction in the number of patients being able to be passed on (from emergency units) in a timely fashion. And we still see exit-blocks.
"It means staff aren't doing what they've been trained to do in emergency medicine. They are now doing the additional things of looking after for longer because of the delays.
"There are patients waiting over four hours a lot of the time. So those patients have to be treated as a ward-based type of patient. The statistics show some patients spend over 12 hours in the department and some over 24 hours."
Dr Roop, a consultant in Wrexham, warned that winter pressures including flu cases could add to the problem: "Absolutely every emergency department (in Wales) is on the edge - we are ever so close to patients becoming really poorly in our departments and that could have a knock-on effect and having disastrous outcomes."
He told the BBC Radio 4 Today programme: "Emergency departments should be able to deal with the workload that they have but the problem lies with the exit blocking within the emergency departments - that's patients who are unable to move out of emergency departments on to their designated wards at the appropriate time."
Part of that was due to the "delayed transfer" of care, he said.
"Patients are unable to get out of the hospital in a timely fashion either to a community setting or back to their homes because of the inability to get social care set up for them."
A Welsh Government spokesman told the BBC: "There was a 23% reduction in the number of people who spent more than 12 hours in emergency departments in December compared to November but we know there is more work to do.
"We expect all health boards to ensure they have the right mix of staff to ensure services are safe, sustainable, and to ensure patient experience and outcomes are optimised."