Author Jean-Louis Beaucarnot traced Sarkozy’s and Hollande’s ancestors through the centuries and claims they share a common relative called Claude Labully-Berty.
Both Hollande and Sarkozy are running in the presidential elections in April.
Labully-Berty worked as a farm labourer in the French Alps. Beaucarnot claims Sarkozy is descended from his son Pierre, and Hollande can trace his ancestry to his son Claude.
Last year, Beaucarnot had claimed that the rivals’ maternal grandmothers both lived in the French town of Lyon and had ancestors in the region of Savoy in the Alps.
Beaucarnot adds an intriguing anecdote to the story in his book The Political World. The Labully name later rose to relative fame when a pastry maker and a member of the Labully family created a breast-shaped cake stuffed with sugar-coated almonds.
The "Labully cake" was baked in honour of Sainte-Agathe, a Christian martyr, whose breasts had been cut off.
Neither Sarkozy nor Hollande have commented on their recently discovered ties.