A public consultation on proposals to merge seven ambulance services into one trust is expected to be held this year.
The final touches are being put to a regional ambulance review which could see Sussex Ambulance Service NHS Trust joining forces with six others in the South-East.
The NHS South-East office and Government ministers are likely to allow a consultation on the proposals to go ahead and a decision is expected shortly.
The other counties covered by the new trust would include Berkshire, Oxfordshire, Hampshire, Surrey, Essex and Kent.
Union leaders are unhappy with the proposals and say they are unsure whether they will benefit patients.
A spokesman for Unison said: "We would need to be sure that services are not affected in any way and patients get the quickest and best facilities possible.
"There are some concerns among staff that ambulances would not be able to cover all the counties if the move is given the go ahead."
Sussex managers have spent the past few months trying to find ways of providing a faster and more efficient service.
Crews have struggled in the last year to meet the national Government standard of responding to 75 per cent of emergency calls within eight minutes.
At its lowest point, at the beginning of last year, crews were arriving on time in just over half of calls.
However, the introduction of new initiatives such as the NHS Direct helpline has helped to improve response times so they are now just under 75 per cent.
But the service admits its main problem is still recruitment, with particular shortages in the Brighton and Crawley areas.
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