LETTERS sent to a female Victorian serial killer as she was held in Durham jail, have today been sold at auction.

Mary Ann Cotton is believed to have murdered as many as 21 people – mostly members of her own family and is labelled Britain’s first serial killer.

She was hanged after being found guilty of murdering her stepson in 1873 but is thought to have also poisoned three of her four husbands, possibly as many as eight of her own children, seven stepchildren, a lover, a friend and even her own mother.

The bundle of Victorian letters, some of them simply addressed to “Mrs Cotton, Durham Gaol” were today sold at Tennants Auctioneers in Leyburn for £1,050.

The letters are from friends, her solicitor and other lawyers representing her and another promises money raised by a committee to pay for her defence. One, from one of her lodgers, describes how her solicitor had gone to the house and taken her belongings, including carpets, knives and forks, to sell to cover her legal costs.

Cotton, branded the Black Widow, was hanged at Durham Prison.

Auctioneer Steve Stockton said: “It’s pretty gruesome but there’s plenty of interest in these horrific people and the darker side of human nature - she was incredibly dark.

He added: “The letters show at the time she had some friends and there were some supporters trying to help her.”

It is thought Cotton’s murderous habits went unchallenged for decades.

She was born in 1832 at Low Moorsley, near Sunderland, now Houghton-le-Spring. When she was eight, and living in Murton in County Durham, her father fell to his death down a mine shaft at Murton Colliery.

She married when she was 20 to a colliery labourer William Mowbray. They had eight children, seven of whom died of gastric fever, before her husband also died of an intestinal disorder.

She went on to marry three more men, all of whom were registered as dying of gastric disorders, as did the children she went on to have.

Cotton became so complacent she would cash in life insurance premiums immediately after her family members had died.

“Child mortality rates at the time were horrific – it wasn’t uncommon for there to be child deaths in the family – but she was a prolific murderer. People were dropping like flies around her and she would go straight to the insurance office and cash in her “winnings”, said Mr Stockton.

“It was the press that picked up on this woman who was leaving a trail of dead bodies.”

Her crimes caught up with her when her seven-year-old stepson, Charles, from her fourth husband Frederick Cotton died and suspicions were raised.

Journalists on North-East papers then investigated and found out about her dead husbands and children.

The doctor who examined Charles had kept samples which tested positive for arsenic.

She was arrested and taken to Durham prison.

She gave birth to a baby while awaiting trial and a week before her execution gave the little girl, Margaret, to a couple for adoption.

One of the letters sold is from the couple informing Cotton that her baby was doing well.