After being hit by issues affecting phone lines and IT systems, a hospital was forced to redirect ambulance patients to other locations.
The Royal Sussex County Hospital in Brighton was unable to receive emergency patients overnight, according to the hospital's trust.
In case of a medical emergency, the trust requested that patients only dial 999.
The trust tweeted on Sunday that the problems had also affected the Princess Royal Hospital in Haywards Heath.
The majority of the IT and communications systems were being brought back online on Sunday afternoon, according to University Hospitals Sussex NHS Foundation Trust.
A spokesman stated: "That progress has resulted in the agreement to stop diverting ambulances from Brighton and Haywards Health.
Despite the progress, he added, "work is still being done to reactivate some of the remaining systems and to troubleshoot potential issues during the day (Sunday)".
A few frontline teams were still being impacted by the outages, he added.
"We ask that people use NHS services prudently moving forward and only turn to emergency services when absolutely necessary.
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