Animal rights activist Barry Horne, who is on hunger strike at Full Sutton Prison is, now so weak he is "giving cause for concern".
Mr Horne is in his 46th day without food, although he is taking liquids.
A Prison Service spokesman said because of his condition, the prison had arranged for him to be seen by a specialist from York District Hospital.
The doctor had visited him in prison, where Horne remains today.
He is serving an 18 year sentence for lighting fires in protest at what he claimed was infringement of animal rights.
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