Downton Abbey star Joanne Froggatt to play real-life serial killer in ITV drama Dark Angel

Froggatt's character Mary Ann Cotton poisoned more than 20 people

Joanne Froggatt as Mary Ann Cotton in upcoming ITV drama Dark Angel (ITV)

Joanne Froggatt is set to play a Victorian murderer dubbed 'Britain's first serial killer' in a new two-part ITV drama.

The Golden Globe-winning actress, known for playing Lady Mary's maid Anna Bates in Downton Abbey, has been cast as Mary Ann Cotton in Dark Angel.

Cotton was found guilty of poisoning her three husbands and hanged in 1873, with some historians estimating that she killed over 20 victims.

The show, based on this horrifying true story, will see Cotton struggling with poverty and longing for a better life before using her ruthless nature to better her financial and social standing through murder, fraud, adultery and bigamy.

Cotton travels around North East England, insinuating herself with various families before killing them, stealing their money and moving on.

Filming is scheduled to begin in North Yorkshire and County Durham this August with further casting to be announced beforehand.

Victoria Fear, ITV's controller of drama, promises an "extraordinary and chilling story".

"We're delighted to have an actress of Joanne Froggatt's calibre in the lead role," she said. "The combination of a tautly written script, an outstanding cast and great producers in World Productions make this a really exciting addition to the ITV drama slate."

Froggatt said earlier this year that she would like her next role to be "someone really evil" - sounds like her wish has been granted.