Prepare yourself for the most disappointing news of the week — the royal baby will not be Jewish.

Hopes for a kosher birth were raised when The Times published a letter from no less an authority than the former BBC court correspondent Michael Cole, claiming that the Duchess of Cambridge had identifiable Jewish roots.

Mr Cole had looked at the family tree of the duchess’s mother and was pretty confident in his assertion. “Carole Middleton is the daughter of Ronald Goldsmith and Dorothy Harrison, both Jews. The parents of Dorothy were Robert Harrison and Elizabeth Temple, both Jews. Elizabeth was descended from the Myers, a distinguished 19th-century Jewish family.”

He concluded: “The Duchess of Cambridge is a Jew on her matriarchal side, and therefore her baby will be a Jew.”

But now a leading genealogist has poured cold water on suggestions that a Jewish king or queen will ascend to the throne in the not too distant future.

“It’s nonsense,” said Doreen Berger, the chairman of the Jewish Genealogy Society (JGS). “I have been researching Kate Middleton’s ancestry since it looked like she was getting engaged to Prince William.

“I’ve looked back as far as it’s possible to look back and she doesn’t have a Jewish link at all — it’s just not true. I’m 100 per cent sure.”

She added that Mr Cole was “confused. The names — Myers and Goldsmiths — are shared by non-Jews as well as Jews. Carole Middleton’s ancestors were a coal miner and a carpenter and they were not from Jewish areas.”

Ms Berger said her research was definitive but she acknowledged that it would not prevent people clinging to the idea that Prince William had married an authentically Jewish princess.

“I don’t know where it came from but it keeps coming up. It’s a hard rumour to stop.”