More than 10,000 A&E patients at the Calderdale and Huddersfield Trust waited more than four hours to be seen last year, figures reveal.
Medical experts have blamed a combination of staff shortages, a lack of funding, and increased demand for rising waiting times across England, and said a no-deal Brexit "would only exacerbate these pressures".
NHS data shows that A&E patients at Calderdale and Huddersfield NHS Trust were left waiting more than four hours on 13,214 occasions in 2018-19.
It means that only 91 per cent of around 152,000 attendances were admitted, transferred or discharged within four hours of arrival – below the NHS’s target of 95 per cent.
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The proportion of patients seen within the target time at the trust has dropped in recent years – the figure stood at 95 per cent in 2013-14.
NHS bosses recently unveiled plans to scrap the four-hour standard – introduced in 2004 – after arguing it was outdated due to the changing nature of emergency care.
It proposed replacing it with four new targets, including a focus on the most critically ill and injured, and measuring the average waiting time for all patients.
The performance of the Calderdale and Huddersfield Trust last year reflects the trend across England – just 88 per cent of emergency attendances were seen to within four hours in 2018-19, compared to 96 per cent five years earlier.
Tim Gardner, senior policy fellow at the charity the Health Foundation, said “huge” efforts by NHS staff saw more people treated in time last year.
But he said the trend towards longer waits is likely to continue, as hospitals grapple with rising demand, a workforce crisis and continued underfunding.
He added: “A no-deal Brexit, which appears increasingly likely, would only exacerbate these pressures.
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“Staffing shortages would be intensified, driving up demand for hard-pressed services, disrupting supplies of medicines and other necessities, and stretching the public finances which pay for health care.”
Dr Simon Walsh, the British Medical Association’s emergency medicine lead, said it was particularly concerning that waiting times had increased in major trauma units, where many of the most critically sick or injured patients are treated.
He described the Government’s recent spending commitment for the NHS as "disappointing", adding: "It’s clear that significantly more investment is needed across the board to turn things around, both for our hardworking staff and the communities they serve.”
A Department of Health and Social Care spokeswoman said: “We’re backing the NHS with £1.8 billion for world-class facilities to improve frontline patient care across the country, on top of our historic commitment of £33.9 billion extra taxpayers’ money every year by 2023-24 – the largest and longest cash settlement in the history of the NHS."
An NHS spokesman said A&E staff across England assessed and treated 3,400 more people within four hours every day last year compared to five years ago.