A syphilitic, mummified corpse found buried in the Swiss city of Basel has been identified over 40-years after it was discovered – and it turns out the 18th-century woman was an ancestor of Boris Johnson.

Genealogists identified it as Anna Catharina Bischoff, the great-great-great-great-great-great-grandmother of the Foreign Secretary.

Her body was discovered in 1975 during renovation work on Basel’s Barfusser Church.

Discovered in front of the altar, the woman was dressed in good-quality clothes and clearly well fed, so researchers knew it belonged to a wealthy lady.

They also established she had been poisoned by mercury, a standard treatment for syphilis between the 15th and 19th centuries. High levels of the toxic substance preserved her remains.

But there was no gravestone to indicate her identity.

It remained a mystery until last year, when scientists used state-of-the-art methods to extract DNA from the corpse’s big toe.

The material was compared with that of living members of a prominent Basel family known to have ancestors buried at the church.

An artist’s impression of how researchers believe Anna Catharina Bischoff may have looked (Basel Natural History Museum)

The research confirmed the woman was Bischoff. Born in Basel in 1719, she spent most of her life in the French city of Strasbourg.

But before her death in 1787, she returned to Basel for syphilis treatment.

Once her identity had been established, genealogists set to work tracing her family tree, which they found stretched as far afield as the US.

They learned she had seven children, but only two survived childhood. Her only daughter was Anna, who married Christian Hubert Baron Pfeffel von Kriegelstein.

Five generations later, Marie Luise von Pfeffel wedded Stanley Fred Williams, whose daughter Yvonne went on to marry Osman Wilfred Johnson Kemal.

Their son, Stanley Johnson, is the Foreign Secretary’s father.

Boris Johnson has previously said he is ‘the product of many countries’ (PA)

The Foreign Secretary has not commented on the discovery, but is known for his international ancestry.

He once told the BBC genealogy programme Who Do You Think You Are? that he was “the product of many countries”, describing his von Pfeffel ancestors as “posh toffs”.