More than 1,500 A&E patients at Calderdale and Huddersfield NHS trust waited four hours or longer to be admitted, transferred or discharged in January, figures show.
These figures formed part of the worst A&E waiting times across England since records began, despite lower levels of flu and norovirus than last year.
NHS leaders said the figures show the “true scale of this crisis facing our systems”.
At Calderdale and Huddersfield NHS Foundation Trust in January there were 12,709 emergency attendances.
Of those, 1,530 patients waited more than four hours to be admitted, transferred or discharged.
The NHS’s target is for 95 per cent of patients to be dealt with in four hours, but at Calderdale and Huddersfield NHS trust, it was just 88 per cent.
This was worse than January 2018, when 91 per cent of patients were seen within four hours.
Across England, the number of patients waiting more than four hours was more than 320,000 in January 2019, with just 84 per cent being treated, admitted or discharged within the target time. This is the worst performance since comparable records began.
The last time trusts across the England hit the 95 per cent target was July 2015.
Royal College of Emergency Medicine president, Dr Taj Hassan, said: "Sadly the situation afflicting our emergency departments has become seemingly normalised with a 'chronic crisis mode' that does not allow staff to deliver the quality of care they would wish and patients should rightly expect.
"The evidence on resultant crowding in departments is clear - it adds to the risk of harm to patients leading to excess deaths and disability.
"These figures make clear the true scale of this crisis facing our systems."
An NHS England spokesman said emergency departments had faced "real pressure, particularly over the past few weeks".
He added: "The health service has seen improved performance since the start of December compared to last winter, with thousands more people successfully treated in A&E within four hours than last year.”
Health think tank the Nuffield Trust's chief economist, Professor John Appleby, said the figures show "the NHS is fighting a losing battle in trying to meet its commitments to provide timely healthcare".
"There is a risk that we lose sight of these problems as Brexit distracts us, or become numbed as we forget the last time targets were met," he said
"But this situation has a serious impact on hundreds of thousands of patients, and will be demoralising for many staff."
NHS England is considering scrapping the 95 per cent four hour A&E waiting time target, to give serious injuries and illnesses more priority.