Ambulances are being diverted and people are being told to avoid A&E after a critical incident was declared at two Sussex hospitals.
Significant phone and IT problems are affecting the Royal Sussex County in Brighton and the Princess Royal in Haywards Heath.
"The disruption is widespread, and frontline teams are affected," said the local NHS Trust.
University Hospitals Sussex advised people on Sunday evening not to attend casualty unless it's a lifesaving emergency.
Patients in ambulances were being diverted to alternative hospitals.
St Richard's Hospital in Chichester and Worthing Hospital are impacted "to a lesser degree" so are helping the two hospitals affected, said the trust.
It warned that people trying to phone in wouldn't be able to get through.
An update at 11.30pm on Sunday said progress had been made in restoring IT systems but it was "still an ongoing issue".
Anyone who does not need an emergency department is asked to call 111 for help or advice.
The cause of the systems outage is so far unclear.
(c) Sky News 2024: Critical incident declared at two Sussex hospitals as people warned to avoid A&E
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