Paramedics say patients' lives are being put at risk because parts of Sussex have no ambulances at night.
They claim one person has already died after waiting more than 20 minutes longer than usual for an ambulance to arrive.
They say ambulance bosses are "playing with people's lives" and warn people will die unless the system is changed.
Under the system, which paramedics say has been operating for several weeks, ambulance crews on duty between midnight and 7.30am are concentrated in larger and densely-populated areas like Brighton and Hove or Crawley.
Staff at Sussex Ambulance Service NHS Trust said this meant outlying rural areas like Horsham and Midhurst did not have immediate cover.
One worker was so worried, she contacted our news desk.
She said: "They are playing with people's lives and people will die. This is not a game.
"I am a paramedic and my colleagues and I feel the public has a right to know.
"It's a lottery and where you live will determine if you get an ambulance within the Government standards of eight minutes."
Government regulations state emergency Category A 999 calls should be answered within eight minutes in 75 per cent of cases.
The service has struggled to meet those targets in the past but it is currently reaching slightly less than 74 per cent of calls.
Staff claim rural areas at night are falling into the last 25 per cent with people waiting well over the eight-minute limit.
The paramedic said: "They are taking ambulances out of areas for hours at a time and I am talking big areas. Last Saturday night there was only cover on the coast and Crawley. Nowhere else.
"There was an incident where there was a cardiac arrest victim who was waiting at least 20 minutes longer than normal for an ambulance to arrive, due to this new policy. He is now dead.
"How many more will die? I don't want to see people die unnecessarily."
Trust director of operations Trevor Anderson said there were on average about 16 to 18 emergency calls between midnight and 7.30am and the vast majority were in busy areas such as Brighton and Hove and Crawley.
He said: "In an ideal world, we would have an ambulance on every street corner at all times but that is not the case so we have to make maximum use of the resources we have.
"We have thoroughly analysed the 999 calls we have received, researched where they came from and at what times. Looking at those results, we have acted accordingly and tried to place ambulances where there is more likely to be a call.
"It is about making the best use of what resources we have and placing ambulances in areas where we believe, based on the evidence of all the calls we have analysed, they will be able to get to people more quickly.
"However, there are occasions throughout the whole of Sussex where people will wait for more than eight minutes irrespective of whether it is a rural or town community because of demands elsewhere."
Mr Anderson said the trust had increased the number of ambulances on the road at night and there were more now than ever before.
Colin Rhodes, a Sussex Unison branch secretary, said he had had several calls from staff worried about the service.
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