The popular TV judge hit back after fans of the BBC 2 show suggested a flirtation between him and the 21-year-old former model.
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Speaking to the Radio Times, Paul, 47, fiercely denied the claims — and even went on to insist he thought Ruby's competitor Kimberley Wilson is "far prettier".
"Personally I think Kimberley's far prettier. With all the love in the world, Ruby's not my type," said the star, who split from his wife of 15 years in the summer.
"But she's a great baker, one of the most talented bakers who has walked in that tent."
Paul, who judges the bakery competition with Mary Berry, dismissed the suggestion that Ruby's looks had improved her chances on the show.
"My job is to judge what goes on the plate. Always has been, always will be," he added.
"I don't judge the person — Mary will back me up on this — because it's not only one judge going this. It's two. And we both say the same thing."
Ruby herself said she paid no attention to accusations of favouritism.
"It's easy to brush it off because I know how it was in the tent — and their was no favouritism," said the student, who is studying for a philosophy degree.
"It was all very fair so that doesn't get to me very much."
The Great British Bake Off continues Tuesday, 8pm on BBC2.