A syphilis-ridden 18th century mummy dug up from a Swiss church is the great-great-great-great-great-great-great grandmother of Boris Johnson, it has emerged.

The body was uncovered in 1975 during renovations on Basel's Barfusser Church, but her identity has remained a mystery for more than 40 years.

Known as 'Switzerland's most famous mummy', the woman was buried in front of the altar, and found wearing expensive clothes, with no signs of malnutrition, suggesting she had been wealthy.

The body survived in a mummified state because she had died of mercury poisoning which had preserved the corpse. But there was no gravestone, or plaque to say who she was, or even when she had been interred.

Scientists from the Natural History Museum of Basel initially thought that the body was from the 16th century because of the wooden coffin, but a chance discovery of new archives last year, revealed that mummy had been discovered once before, in 1843, and linked to the wealthy Basel family the Bischoff's.

To be sure, researchers extracted DNA from the mummy's big toe and compared it to genetic material from a modern relative Rosemary Probst-Ryhiner.

Not only was it a match, showing that the woman must have been a Bischoff, but further archival research showed she was Anna Catharina Bischoff, who died in 1787, a direct maternal ancestor of British Foreign Secretary Boris Johnson.

Anthropologist Gerhard Hotz, of the Natural History Museum Basel, said: "Thanks to state-of-the-art analysis, we were able to solve the puzzle after years. What was her name? Who was she? And how did she die?

"The final proof needed a relative."

Anna Catharina had seven children, including Anna who married Christian Hubert Baron Pfeffel von Kriegelstein. Her great-great grandaughter, Marie Luise von Pfeffel married Stanley Fred Williams and their daughter Irene, who Mr Johnson always referred to as 'Granny Butter' wed his grandfather Wilfred 'Johnson' Kemal.

The couple changed their name from Kemal to Johnson amid growing tensions with Turkey.

In the BBC programme 'Who Do You Think You Are?' Mr Johnson said his grandmother had spoken about her aristocratic ancestors, but nobody in the family believed her claims. He described the von Pfeffels as 'posh toffs'.

However, his Swiss relative had a more humble life. Anna Catharina married a church minister and spent most of her adult life living in Strasbourg. Researchers believe she may have contracted syphilis while caring for patients with sexually transmitted diseases while working in the French town.

On hearing of his new relation, Mr Johnson tweeted: "Very excited to hear about my late great grand 'mummy' - a pioneer in sexual health care. Very proud."

After the death of her husband she returned to Basel, and her body showed she had undergone extensive mercury treatments, which were thought to be a cure for syphilis at the time. However, the treatment proved more deadly than the disease, and it is probable that it killed her, scientists believe.

Yet the mercury also preserved and mummified her body, allowing researchers to find out her identify.

© Daily Telegraph London

Telegraph.co.uk