The fire broke out at Crawley Hospital in West Green Drive, Crawley at 13.36 pm.
It's believed to be in a sixth floor store room. There are no reports of casualities.
50 firefighters are involved with tackling the blaze.
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Windsor-based Centrica, the parent company of British Gas, is cutting six thousand jobs.
Thursday afternoon's weather for the east of the region
Thursday afternoon's weather for the west of the region