The system is part of the trust's control centre software and is vital in enabling ambulances to respond to the 600 to 900 emergency calls it receives every day.
After two weeks of intermittent faults in its control centre's despatch and mapping software, which started on 22 February, Sussex Ambulance Service decided to switch to pen and paper. This was "through choice" and "in a controlled way, as a precaution, while we try to isolate the fault", said a spokeswoman.
The despatch system is eight years old, one of the oldest in the UK. It handles the despatch and location of ambulances on accident and emergency calls and is part of a larger IT system that uses satellite-based vehicle tracking and digital mapping.
Sussex Ambulance Service uses Terrafix, a digital communications and mapping system that encodes data and transfers it through a secure radio channel. It links to satellite systems in the vehicles and a separate computer-aided despatch system, which is where the fault developed.
On several occasions the intermittent fault caused screens to freeze, the system to breakdown and links with other computer systems to fail.
The trust is bringing in external IT companies to identify where the fault is, but said it would cost more than £1m to replace the ageing kit.
David Davies, a spokesman for public services union Unison, said the problem left ambulance crews unable to respond to emergencies as well as they could if the computers were working. He added that if performance was affected, which he said was likely, there would be an implication for people with life-threatening injuries and urgent medical problems.
But the trust stressed that the manual system, although not ideal, was safe, and allowed it to despatch and locate ambulances in the same way as before. "Staff practice using a manual pen and paper system as a fall-back. Phone and radio systems are unaffected, so we can receive calls in the same way," said the spokeswoman.