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The never-seen-before letters of a woman believed to be Britain's first serial killer have emerged for sale 143 years later.

Mary Ann Cotton was hanged on March 24, 1873, after being found guilty of murdering her stepson, but she was believed to have had as many as 21 victims.

While there wasn't enough evidence to convict her, Cotton is thought to have used arsenic to poison and kill three of her four husbands, possibly as many as eight of her own children, seven stepchildren, her mother, a lover and an inconvenient friend.

Now letters cleared from her cell by the matron of Durham Jail are being sold for the first time at Tennants Auctioneers in Leyburn, North Yorkshire.

(Image: BNPS)

The letters offer an insight into her final days.

Mary Ann Cotton hasn't retained the notoriety of killers like Jack the Ripper or Harold Shipman, but a renewed interest in the female mass murderer was sparked a few years ago with the publication of a book by criminology professor David Wilson.

A TV drama called Dark Angel starring Downton Abbey's Joanne Froggatt followed last year.

She first married when she was aged 20 in 1852 and within 20 years she had moved around the north east leaving a trail of dead loved ones behind her.

(Image: Newcastle Chronicle)

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

Journalists caught on to the story and found out about her dead husbands and children.

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

Cotton was arrested and taken to Durham Jail.

Before her arrest she had worked as a nurse for John Quick-Manning, who was recovering from smallpox, and became pregnant by him.

Her trial was delayed for several months so she could give birth.

She nursed the baby, Margaret, in her cell until all her avenues of appeal had been exhausted and a week before her execution she gave the infant to a couple she knew, William and Sarah Edwards, who adopted the girl.

One of the letters is from the Edwards, informing Cotton how the baby is doing, which seems ironic considering she was accused of murdering so many of her children and stepchildren.

The letter, which was sent on March 20, just four days before she was executed, was addressed to 'my dear friend' and said: "I think it is my duty to write you a few lines and let you know how your little baby is getting on.

"We had many hundreds of visitors to see it and they say it is a fine one.

"Poor little thing it had a pretty good night but a better day. But you must not let it trouble your mind for it will be alright by god's help.

(Image: Newcastle Chronicle)

"I may just say Mrs Edwards hopes we will all meet in a better world."

Another is a letter from William Lowrey, who had been Cotton's lodger when she was arrested. It describes her solicitor George Smith going to the house and taking her belongings to sell to cover her legal costs. It mentions him taking "the bed, carpets, knives, forks" and selling them for £13.

Others are from her solicitor and other lawyers about representing her and another promises money raised by a committee to pay for her defence.

The letters and photograph have an estimate of £700, but Steve Stockton from the auction house said they could sell for several thousands.

He said: "It's a very interesting story and has a great line of provenance.

"The vendor's ancestor was the matron at Durham Jail at the time Cotton was there and the presumption is that she cleared out her cell after she was hanged.

"The items have been in the family ever since and have never been on the market.

"They've been hidden in drawers over the year, never seen in public until now.

"These are all letters written to her in the lead up to her trial. There are a few about her representation for the trial.

"She was pregnant when she was in jail and interestingly there is a letter from the couple who looked after her child telling her the baby was okay, which is quite ironic considering she was believed to have murdered most of her children.

"These items have got a great story behind them and there's a real human interest.

"It might be a bit gruesome and macabre but people are fascinated by serial killers and mass murderers.

"We had some letters written by Cotton up for sale several years ago and those made £3,000, so it's difficult to predict what these will go for."

The letters will be sold on September 21.