Mary Ann Cotton is thought by historians to have killed as many as 21 people - including 11 of her own children

Two decades before Jack the Ripper became Victorian London's most feared serial killer, a seemingly harmless County Durham housewife had already poisoned her way into the annals of British crime history with a horrific crime spree.

Experts are divided on how many bodies Mary Ann Cotton left in her wake as she posed as a kind and caring wife, mother, and sometimes widow.

Between 1860 and 1872 she is believed to have put an end to three husbands, a lover, and up to 11 of her children.

She was sent to the gallows in 1843 at the age of 40.

Some historians think the death toll she caused may be as high as 21 - all of them victims of a cruel and scheming temptress who killed with impunity using arsenic to make it look they had died from fever.

And yet a researcher who has spent the last 30 years investigating Cotton's conviction has said the case is not so clear-cut - saying authorities took shortcuts and used hearsay to get a conviction, and said presenting her as a 'psychopath' is unfair.

While Jack the Ripper remains one of the most notorious serial killers ever to baffle investigators, Cotton remains comparatively unknown.

Until now, that is.

A new ITV drama, starting on Monday night, will star Joanne Froggatt, of Downton Abbey fame, in the lead role. The two-part series, Dark Angel, will examine the life of woman believed to be the country's first serial killer.

Sinister: Cotton killed her victims by poisoning them, and historians believe she could have killed 21 people, including 11 of her own children

Cotton will be played by Downton Abbey star Joanne Froggatt in upcoming two-part drama Dark Angel

Historian Ian Smyth Herdman has called for the public to take the drama with a pinch of salt though.

He told MailOnline: 'It is sickening that over the years MAC's family have to suffer hearsay, rumours and accounts of MAC's as being accurate, when I'd say 99 per cent of the information is nothing more than rehashed accounts.'

And Mr Smyth Herdman added: 'The fact that the drama will be aired on Halloween is yet another indication that it cannot be taken seriously, as the timing is pathetic and an indication of hyperbole and sensationalism'.

The case against Cotton: It is long believed that she was responsible for up to 21 murders

Cotton denied her guilt until her day, but was convicted of murder after a three day trial at Durham Crown Court

The first part of the drama will be screened on ITV at 9pm on Monday night

He claimed that prosecution's star witness, a chemist called Thomas Detchon, was able to pick Cotton out in an identity parade because he had been shown a photograph of her beforehand.

I think she was a troubled person, and I believe my research shows that she suffered from depression, but was certainly not - as some authors describe - a psychopath. Ian Smyth Herdman

Far from being a cold-blooded killer, he believes, she was one of Sunderland Infirmary's best fever nurses, with a string of experts lining up to be character references at her trial.

Mr Smyth Herdman, who runs a website dedicated to the case, said: 'I believe like most Victorian women who had child after child, her life was one of difficulty, she made the best of the situation.

'Deaths in that era were frequent, the mortality age being very low for adults too.

'I think she was a troubled person, and I believe my research shows that she suffered from depression, but was certainly not - as some authors describe - a psychopath.'

He said his research has convinced many - including students interested in the case - that it was miscarriage of justice.

Cotton was locked up at Durham Prison after her conviction, but continued to protest her innocence until the day she died

At the time of her death, the Newcastle Chronicle described her as a ‘monster in human shape’.

David Wilson, professor of criminology at Birmingham University, who wrote a biography of the convicted killer, is convinced of her guilt.

Professor Wilson, author of the book Murder Grew With Her: On The Trail Of Mary Ann Cotton, Britain’s First Serial Killer, wrote that even 'crime aficionados' know little or nothing of her.

The murderer, he observed 'has been reduced to little more than a chilling bedtime story and a Northern nursery rhyme: "Sing, sing, oh, what can I sing? Mary Ann Cotton is tied up with string. Where, where? Up in the air, sellin’ black puddens a penny a pair."'

Professor David Wilson, who wrote a biography of Cotton and who makes a cameo in the series, wrote: 'It is hard not to believe that there was some element of enjoyment at the control she exercised'

Professor Wilson, who makes a cameo in the series as Cotton's lawyer, said he had uncovered toxicology reports at Leeds University which prove her guilt.

Born Mary Ann Robson in 1832 in Low Moorsley, a village outside Sunderland, she married first husband William Mowbray in 1852.

The couple and their family moved to Cornwall, but returned to the North East in 1860 - which is when the killing began.

Her style, Prof Wilson observed, was very simple.

'Mary Ann would find a man with an income, live with him until it became inconvenient, and then murder him,' he wrote.

The Durham home where Cotton was arrested for the killing of her stepson, seven-year-old Charles Edward Cotton

It has long been held that she killed her husbands in order to claim insurance money

'Numerous children – no one knows how many – were dispatched with the same callousness.'

Mary Ann would find a man with an income, live with him until it became inconvenient, and then murder him Professor David Wilson

She would dispatch her victims with arsenic, which brought on symptoms including vomiting, diarrhoea and dehydration - which could be easily disguised as gastroenteritis.

All of her four children with Mowbray, three died young, while her first husband perished in 1835.

It was this that provided the motive for her killings, investigators found - she was entitled to a payout of £35 from British and Prudential Insurance.

She married her second husband, George Ward, in August 1865. He was dispatched a year later, entitling her to a second insurance payout.

Next came widower James Robinson, who became her husband in 1867.

Yet he was to survive his marriage to the serial killer, throwing her out after discovering she lived on the streets, forcing her to live on the streets.

But she was not to be single for long, turning her attentions to Frederick Cotton. She committed bigamy to tie the knot in March 1870, and he was dead by the end of the year.

At this point her long-term lover, Joseph Nattrass, became her lodger. Shortly after revising his will in her favour, he was dead as well.

Letters written by Mary Ann Cotton were put up for auction last month amid heightened interest in the case

Cotton protested her innocence until her death, writing a string of letters from prison arguing that she was not a killer

Her neighbour Jane Hedley would later testify at Cotton's trial at Durham Crown Court that Nattrass had died writing in agony.

Prof Wilson wrote: 'It is clear from Jane Hedley’s account that, by this stage at least, Mary Ann had the confidence to kill right under the noses of the doctors.'

And he continued: 'It is hard not to believe that there was some element of enjoyment at the control she exercised – that she was, in other words, a psychopath. I believe she would have enjoyed holding down Nattrass as he died writhing in agony.'

Despite protesting her innocence, Cotton was hanged in 1873, having been convicted in a three-day trial at Durham Crown Court

Also convinced of Cotton's guilt is retired Supt Stephanie Yearnshire, who told The Mirror ahead of the new series: 'This was a woman who was expert at seducing men, she was attractive and intelligent as well as being calculating and ruthless.'

It was the poisoning of her stepson, seven-year-old Charles Edward Cotton, that led to her undoing. His death was ruled to have been caused by natural causes, but by this stage her trail of victims had been noticed, and local gossip prompted a police investigation.

After a three day trial, she was executed on March 24, 1873, despite writing numerous letters to newspapers protesting her innocence.

Poisoning her victims meant there was little chance of her crimes being discovered, her trial heard

Mr Smyth Herdman said: 'MAC was only ever convicted of one murder, that of her stepson Charles Edward Cotton. Without this conviction the other three cases attributed to her at Durham Crown Court would collapse.'

The hanging itself was a horrific debacle -the drop below the trap door was too short, and the hangman was forced to press down on her to finish the job.

Mr Smyth Herdman has an unpublished book which he hopes will further understanding about Cotton, and also plans to produce a play about the convicted murderer.

Despite her protestations of innocence, Cotton was hanged in 1873 and is believed to be Britain's first serial killer

With attention set to focus on Cotton with the new series, her descendants are keen to avoid the spotlight.

Her great-great grandson told The Mirror, on condition of anonymity: 'It’s such a strange, dark secret for a family to have. To say it came as a shock when my dad told me, quite casually, that she was a direct relative, is an understatement.