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 national archive.

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       In one letter, Marie-Antoinette wrote: “I will finish not without telling you my dear and loving friend that I love you madly and that I can never be a moment without adoring you.”

       In another, von Fersen said: "What will become of us, my dear friend? Let’s think about it. Without you, there is no happiness for me, the universe is nothing without you."

       To unravel the mystery of the content, the researchers applied X-ray fluorescence spectroscopy to analyse the redacted sections of 15 letters.

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       They uncovered censored writing from eight of the letters.

       Anne Michelin, Centre de Recherche sur la Conservation (CRC), MNHN, Sorbonne-University, CNRS, and colleagues conclude that their strategy offers an alternative to existing methods for unveiling redacted content.

       They suggest it may have broader historical and forensic applications, demonstrating the power of combining advanced data processing tools with macroscopic elemental mapping to disentangle superimposed ink and to make hidden content more legible.

       Marie-Antoinette married into the French royal family in 1770.

       She had attempted to flee Paris to Varennes with her family in June 1791 but they were captured and placed under house arrest.

       In September 1792 the Frech monarchy was abolished and replaced with the National Convention.

       Louis and Marie-Antoinette were both found guilty of treason and executed by guillotine in 1793.

       Additional reporting by Maria Ortega.