Paramedics are waiting with patients in ambulances outside hospital because of a backlog in the casualty department.
Crews photographed ambulances outside the hospital on Monday
Ambulance officials blame the problem at Redhill Hospital on the closure of Crawley Hospital's accident and emergency department.
Sussex Ambulance Trust chairman Richard Purchase wrote to hospital managers to say the situation was "clinically unsafe".
The trust which runs the hospitals said the situation was improving.
The A&E department at Crawley Hospital closed at the end of August as part of a £30m overhaul of services with a minor injuries unit set up in its place.
Normally the average time for transfer from ambulance to hospital takes between 15 minutes and half an hour.
Sussex Ambulance Trust said patients have been waiting in queuing ambulances for up to two hours in Redhill.
Mr Purchase said in the letter to hospital management that paramedics were waiting "in excess of two hours" before they could go to other jobs and said the situation "cannot and must not continue".
Crawley MP Laura Moffatt said she was demanding a meeting with hospital managers to find out why the problems were arising.
Dr Frances Mathey, medical director for the Surrey and Sussex NHS Trust, said: "When the ambulances arrive the patients are initially assessed and taken into the accident an emergency department for example if they need urgent resuscitation.
There is now a minor injuries unit at Crawley Hospital but no A&E
"Sometimes the ambulance staff need to stay with them (the patients) and we are addressing that but there is no question of patients not being assessed promptly and given the treatment they need.
"We have had problems. It was particularly bad, from the evidence we have, in the week following the bank holiday.
"It was bad in that week and has now got better and is continuing to get better so the number of delays is reducing day by day."
But Duncan Jones, from Unison, said: "We have evidence from two days ago that the situation has not improved, if anything they have remained the same and are looking like they may get worse in the future."