How many did she kill? Mary Ann Cotton was hanged in March 1873 in Durham after being found guilty of murdering her stepson

A woman believed to be Britain’s first serial killer is feared to have used arsenic to murder 21 people including 11 of her children and three husbands.

Mary Ann Cotton was hanged in March 1873 in Durham after being found guilty of murdering her stepson, but she is said to have killed many more relatives.

And the never-seen-before letters of Cotton that offer an insight into her final days have now emerged for sale 143 years later at an auction on Wednesday.

Cotton - known as the 'Black Widow' - is said to have used arsenic to poison and kill three husbands, 11 of her 13 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.

A renewed interest in Cotton was sparked in 2012 with the publication of a book by criminologist David Wilson, whose research was featured in the Mail.

Update: One of the letters up for auction is from the Edwards couple, informing Cotton how the baby is doing on March 20 - just four days before she was executed

Hopes for the future: The letter 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'

And an ITV costume drama about her called Dark Angel, starring Downton Abbey actress Joanne Froggatt, is expected to be broadcast later this year.

Cotton first married when she was aged 20 in 1852 and over the next few decades moved around the North East leaving a trail of dead family members behind her.

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 investigated and found out about her dead husbands and children. The doctor who examined Charles then found his samples tested positive for arsenic.

Cotton was arrested and taken to Durham Jail. But before her arrest she had worked as a nurse for smallpox sufferer John Quick-Manning 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.

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

Then, 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 couple, informing Cotton how the baby is doing on March 20 - just four days before she was executed.

It 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. 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.

Collection: The letters and photograph have an estimate of £700, but Steve Stockton from Tennants Auctioneers said they could sell for several thousand pounds

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.

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 Tennants Auctioneers said they could sell for several thousand pounds.

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 OK, 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.