Martin Luther was candid about his constipation

The stone room is in a newly-unearthed annex to Luther's house in Wittenberg.

Luther is quoted as saying he was "in cloaca", or in the sewer, when he was inspired to argue that salvation is granted because of faith, not deeds.

The scholar suffered from constipation and spent many hours in contemplation on the toilet seat.

'Earthy Christianity'

The lavatory was built in the period 1516-17, according to Dr Martin Treu, a theologian and Luther expert based in Wittenberg.

We still don't know what was used for wiping in those days Dr Martin Treu

The toilet is in a niche set inside a room measuring nine by nine metres, which was discovered during the excavation of a garden in the grounds of Luther's house.

Dr Treu said there can be little doubt the toilet was used by Luther, the radical theologian who argued for a more "earthy Christianity", which regarded the entire human body - and not just the soul - as God's creation.

The Reformation, which resulted in Europe's Protestant churches, is usually reckoned to have begun when Luther nailed 95 theses to the door of Wittenberg's Castle Church on 31 October 1517.

The theses attacked papal abuses and the sale of indulgences by church officials, among other things.

Structural concerns

Luther left a candid catalogue of his battle with constipation but despite this wealth of information, certain key details remain obscure - such as what the great reformer may have used in place of toilet paper.

"We still don't know what was used for wiping in those days," says Dr Treu. The paper of the time, he says, would have been too expensive and critically, "too stiff" for the purpose.

And while it is probable that the inspiration that led to Luther's reforms occurred on this toilet, it is impossible to prove it beyond doubt, Dr Treu says.

Future visitors to Wittenberg's Martin Luther museum will be able to view the new find, though structural concerns mean they will not be free to test its qualities as a toilet.