The report says the British monarch had five major episodes of prolonged and profound mental derangement while on throne. The King's illness was originally thought to be a psychiatric disorder or acute attacks of a defect brought on by faulty synthesis of a protein.
King George III reigned from 1760 until his death in 1820.
Martin Warren at the University of Kent and his colleagues were investigating exposure to metals in a sample of the King's hair, when they found the high concentrations of arsenic.
The researchers later found from the royal physician's medical notes that the principal compound administered to the King during his illness was emetic tartar. Emetic tartar contains a substance called antimony, which can be contaminated with arsenic.