John Hurt, who mistakenly thought his family hailed from Ireland, actually has Croydon ancestry.
Genealogists on the BBC's Who Do You Think You Are programme made the discovery, which has come as a blow to the actor.
For years Hurt's family believed his great-grandmother was the illegitimate child of the Marquis of Sligo.
But this appears to be a myth, perhaps created by his ancestors to give their family tree a bogus link to the upper class.
The star of The Elephant Man, who moved to Ireland for a time, told the Telegraph: "As far as I was concerned I was Irish.
"My disappointment was that they had managed to prove that the one thing I thought I did have was Irish blood and I haven't got any."
The programme will be shown tomorrow night.
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