The Duke has a direct link with a part-Indian woman called Eliza Kewark which traces back just eight generations, genetic ancestry testing firm BritainsDNA said.
Eliza was housekeeper to William’s fifth great-grandfather Theodore Forbes, a Scottish merchant born in 1788 who worked for the East India Company in the port of Surat, north of Bombay (now Mumbai).
She is the Duke’s great, great, great, great, great-grandmother, according to the analysis of saliva samples taken from the second-in-line to the throne’s relatives.
BritainsDNA, which used a combination of traditional genealogy and cutting-edge science for the study, said the Duke’s genetic link to India runs through the maternal line.
The future king carries Eliza’s mitochondrial DNA (mtDNA), a small piece of DNA inherited mostly unaltered from a mother to her children.
The mtDNA was passed on by Eliza’s daughters and granddaughters directly in an unbroken line to Diana, then on to Prince William and Prince Harry, the researchers found.
The Duke’s heirs are “very likely” to carry a small amount of Indian DNA from Eliza, whose father may have been of Armenian descent, the scientists said.
Dr Jim Wilson, chief scientist at BritainsDNA and a genetics expert at the University of Edinburgh, carried out the research.
He said: “This is a great example of how genetics can be used to answer specific historical questions and uncover fascinating facts about our ancestry.”
Two of Eliza’s living direct descendants, who are third cousins of Princess Diana’s mother, Frances Shand Kydd, were traced using birth, marriage and death records, Dr Wilson said.
Eliza was born in around 1790 and resided in India when it was under the governance of the East India Company. Her surname points to her possible Armenian ancestry, as does the presence of Armenian script in her letters to Forbes, the research added.