A major incident tent had to be set up outside a Norfolk hospital because ambulances were left to queue outside the A&E department for hours.
The mobile treatment area was set up at the Norfolk and Norwich University Hospital after 15 ambulances had to queue up on Monday.
The East of England Ambulance Service said each vehicle had to wait for up to three hours to hand over patients.
The hospital said demand at the A&E department had been high.
Anna Dugdale, chief executive for the Norfolk and Norwich University Hospital (NNUH) said: "We were extremely busy over the weekend.
"We made a decision with the ambulance trust to put the tent up simply as a precaution at about half-past six last night.
"We agreed by quarter-past eight that we didn't need it."
The East of England Ambulance Service (EEAS) said that between 11:00 BST and 20:00 BST on Monday there had been between six and 15 vehicles queuing for up to three or more hours.
Andrew Morgan, interim chief executive of the ambulance service, said the impact on patients and staff was "not acceptable".
"We decided to deploy extra resources from our special operations teams on the grounds of patient safety," he said.
"Patients have to come first in all our decision making. It is not good care for patients to be waiting for a long time to be going into any A&E department."
The Trust said it had worked closely with the hospital and the Clinical Commissioning Group - the group of GPs who have taken over the running of the local primary care trust - to resolve the issue.
At the beginning of March all 17 of Norfolk's ambulances waited more than two hours to transfer patients to the NNUH.
At the time, health minister and North Norfolk MP Norman Lamb said "lives had been put at risk" because of delays in transferring patients from ambulances to the hospital.
The EEAS has been under fire in recent months for poor response times.