Ambulances' three hour wait outside Worcestershire A&E
Paramedics trained to deal with major incidents were sent to a hospital after 10 ambulances were left waiting for almost three hours to hand over patients.
West Midlands Ambulance said its Hazardous Area Response Team was sent to assist at Worcestershire Royal Hospital on Friday.
The service said it has now written to health bosses about the situation.
The trust which runs the hospital said "the weekend has been very busy".
The ambulance service said its HART team surveyed the scene.
But the situation was resolved with an hour and the team did not need to deploy its 'tent', a temporary structure where patients can be treated.
It said, at one point, a total of 14 ambulances were tied-up at the hospital, about two thirds of the 21 ambulances working in Worcestershire at the time.
But, the ambulance service said it only took one more patient than predicted to the hospital between 6pm and midnight on Friday.
A spokesman confirmed it has now written to the Care Quality Commission, the hospital, NHS England and NHS Improvement about the situation.
According to national guidelines, Worcestershire Acute Hospitals NHS Trust is expected to deal with 95% of patients who attend A and E within four hours, but the BBC understands on Friday it dealt with 47% in four hours and 49% on Saturday.
On Friday, it dealt with just 47% in four hours, which rose to 49% on Saturday.
A spokesman for the trust apologised for the delay. He said: "We accept that during what was a period of extremely high demand, we were not able to accept patient handovers from ambulance colleagues as promptly as we would have wanted."