The 77-year-old former Goon collapsed at his home in Shamley Green, near Guildford, Surrey, and a doctor who was called diagnosed the stroke.
Sir Harry, a committed Christian who was a popular presenter of Sunday evening programmes Songs of Praise and Highway, was taken to the Royal Surrey County Hospital in Guildford.
He was later transferred to the private Nuffield Hospital in Surrey.
His wife is understood to be at his bedside.
The comedian turned singer has recently been battling against prostate cancer which was diagnosed shortly before Christmas.
Neighbours at his Surrey home said that an ambulance was called to the house on Friday evening and Sir Harry was taken to hospital immediately.
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