Hundreds of patients have been stuck in ambulances outside A&E as paramedics queued to transfer them into the busy emergency department.
All patients are supposed to be handed over to A&E within 15 minutes - but that target was breached on 937 occasions in a single month at Mid Yorkshire Hospitals Trust.
A huge increase in delays saw 247 handovers take more than an hour in March, on 238 occasions at Pinderfields Hospital and nine times at Dewsbury and District Hospital.
The overall figure rose from just 31 delays of more than an hour in March 2015.
It took more than 30 minutes to transfer patients into A&E on a further 690 occasions in March 2016, the vast majority - some 646 - at Pinderfields and 44 times at Dewsbury. The 30-minute figure was up from 254 a year earlier.
The delays mean paramedics are not free to respond to fresh 999 calls while they wait at A&E.
Mid Yorkshire said measures to speed up handovers included a new assessment team in A&E and a self-handover service for paramedics.
Dr Sarah Robertshaw, head of clinical service emergency medicine, said: “Mid Yorkshire NHS Trust is aware of the increase in delayed ambulance handovers and takes this very seriously.”
NHS targets also mean 95 per cent of patients should be seen in A&E and either admitted to hospital, transferred or discharged within four hours.
But Mid Yorkshire only managed 79.7 per cent in March.
A report to the trust board said: “Of 21,381 attendances in March 2016, 4,343 waited more than 4 hours to be admitted, transferred or discharged.”
Mid Yorkshire had the third busiest A&Es in England, the report said.