Nearly a fifth of patients attending Accident and Emergency (A&E) at Pinderfields, Pontefract and Dewsbury Hospitals are waiting longer than four hours to be seen.
Latest figures from the Mid Yorkshire Hospitals NHS Trust, which runs all three sites, show that just over 82 per cent of people who attend A&E are treated within the government's recommended timeframe.
However, those with the most serious problems are being seen quickly. Almost every patient treated by Type 1 specialists - who deal with life-threatening and life-changing injuries - has been seen on time within the last three months.
At a trust board meeting on Thursday, director of operations Trudie Davies said that A&E's performance had improved on its record from last winter and that staff had worked hard to ensure there were no bed shortages.
But she added that pressures on the service remained, with Pinderfields in Wakefield struggling the most to cope.
Ms Davies said: "Our particular concern at the moment is with A&E and on the wait to be seen.
"Our performance in December was better than the previous December, but it's not to the standard that we would have hoped, especially given the winter planning we put in place.
"We're doing well at getting people out of hospital when they're ready to go home, but we're seeing ongoing pressures on the emergency care standard (four hour target).
"The big pressure is on Pinderfields, rather than Dewsbury and Pontefract. That's where we've seen most of the growth in activity."