More than one in five A&E patients at Mid Yorkshire Hospitals trust waited four hours or longer to be admitted, transferred or discharged in December, figures show.
This was worse than December 2017, and health bosses warned the worst of this winter could still be to come.
There were 17,662 emergency attendances at Mid Yorkshire Hospitals NHS Trust in December.
Of those, 3,696 patients waited more than four hours to be admitted, transferred or discharged.
The NHS’s target is for 95% of patients to be dealt with in four hours, but at Mid Yorkshire Hospitals trust, it was just 79%.
This was worse than December 2017, when 81% of patients were seen within four hours.
Across England, the number of patients waiting more than four hours reduced from 292,860 in December 2017 to 270,171 last December – despite overall attendances increasing.
Dr Nick Scriven, president of the Society for Acute Medicine, said: “There is more demand on the NHS, but that is something those of us on the frontline have been warning of for some time, and it cannot become an acceptable rationale for poor performance.
“My sincere hope is that the so-far mild weather has not lulled our leaders into a false sense of security, and that just because things are better than 12 months ago, this does not make it good, as key targets in preparing for winter were not achieved.”
An NHS spokesman said: “Thanks to the hard work of NHS staff, monthly figures published this morning show the health service performed better for A&E services this December, than December 2017, despite successfully caring for 3.9% more people within the current four-hour target.
“We are now in what can be the most pressured time of year – flu, other winter bugs and adverse weather conditions can all increase, so the situation is being closely monitored.”
Shadow health secretary Jonathan Ashworth criticised the “staggeringly bad” NHS England figures, saying they were a “shocking indictment of nine years of Tory cuts, failure to recruit staff and a botched NHS reorganisation”.
“Patients are waiting longer in pain and distress, with a real risk that their health could deteriorate further,” he said.
“Patients deserve better.”
NHS leaders recently said they were considering scrapping the 95% four hour A&E waiting time target, to give serious injuries and illnesses more priority.