Emperor Charles V's secret code cracked after five centuries

Letter from 1547 reveals Spanish king's fears of a French assassination plot against him

A team of researchers have cracked a five century-old code that reveals a rumoured French plot to kill the Roman emperor and king of Spain Charles V.

Charles was one of the most powerful men of the 16th century, presiding over a vast empire that took in much of western Europe and the Americas during a reign of more than 40 years.