Sections of the letters, written during the French Revolution by the wife of Louis XVI and Swedish count Axel von Fersen, were redacted. The censored content has been puzzling historians for almost 150 years.
But parts of the redacted correspondence have now been uncovered using new technology.
A combination of X-ray and data processing techniques were used to reveal the hidden content.
Researchers said words including “beloved”, “tender friend”, “adore” and “madly” suggest the closeness of the relationship between Marie-Antoinette and von Fersen.
The study also found evidence that the Swedish count censored the letters himself, indicating they were important to him either for sentimental or political reasons.
A report in the US-based Science Advances journal said: “Whether state secrets, escape plans, or evidence of a royal love affair, this presumably sensitive content has been puzzling historians for almost 150 years.
“He decided to keep his letters instead of destroying them but redacting some sections, indicating that he wanted to protect the honour of the queen (or maybe also his own interests).”
The study added: “The choice of vocabulary (beloved, tender friend, adore, madly) attests a particular relationship between Marie-Antoinette and Fersen even if there is an influence of the revolutionary torment, which favours a certain emotional intensity.”
Some of the letters Marie-Antoinette exchanged with von Fersen between June 1791 and August 1792 are held at the French…