Diane de Poitiers was renowned for her youthful looks and porcelain skin and thought the concoction preserved her youth.

Experts say she was up to 20 years older than the king but her appearance made them look the same age. One courtier said she was "as fresh and lovable" in her final years as when aged 30 and had skin "of great whiteness".

She was unusually athletic for the time, keeping in shape by horse riding, hunting and going on daily swims in the river next to the chateau d'Anet, in northern France where she lived.

But her secret was the elixir she drunk every day made up of gold chloride and diethyl ether.

It was one of a host of anti-ageing treatments peddled by apothecaries, along with recipes including spider webs, earthworms, frogspawn and scorpion's oil.

However, French experts writing in the British Medical Journal say the yellow liquid said to harness the powers of the sun and keep her immutable actually slowly killed her at the age of 66.

Three scientists unearthed her bones last year and found the jaw perfectly matched her portrait while her leg bone had a clean break from a documented riding accident.

The breakthrough came when they studied locks of her hair kept at the chateau d'Anet: these were found to contain gold 500 times above normal levels, as well as mercury – used as a "purifier" in the elixir.

Since she was not a queen and did not wear a crown, scientists ruled out the possibility that jewellery had contaminated her hair and body.

Scientists Joel Poupon and Philippe Charlier, who usually works in hospital morgues in Paris, worked together to identify de Poitiers in the recently opened Normandy grave.

"Her hair was much finer than normal, which is a secondary effect of chronic gold poisoning," said Mr Charlier.

"It gives you white skin (from anaemia) and very fragile hair, bones and teeth. She was in this fragile state when she died."

After the king's death, de Poitiers was banished from court by his widow Catherine de'Medici to the chateau in Anet given to her by the king. She died there in 1566 and was buried in a tomb in a specially built funeral chapel.

Her grave was desecrated during the French Revolution and her body flung into a common grave outside the chateau's walls. Her bones will now be laid to rest back in the chateau next year.