A mother killed her six-week-old baby because she thought it was a ‘devil’, a court has been told.

The 26-year-old, who cannot be named for legal reasons, moved from Senegal, Africa to Wallsend, Newcastle last year with her elder daughter to reunite with her husband, Fairfax Media reported.

The mother suffered from a rare mental illness known as Capgras delusion, which caused her to slit her child’s throat last July.

The father found his wife and the baby on a bloodied mattress.

The mother had told forensic psychiatrist Richard Furst that in the weeks after the birth she became anxious and depressed.

‘Weird voices’ called her and she came to believe her child was ‘not my baby, something else I was holding and had to deal with,’ she said.

‘It was quite scary. More like a devil thing I was holding.’

But a court found in September that she did not suffer from the disorder during her pregnancy or the weeks following the birth.

Two days before the incident, she told her husband that she was unhappy but refused to see a doctor.

He then gave her anti-depressants the day after and she was much happier according to her friends.

After giving birth, she told a forensic psychiatrist that she became anxious and depressed.

Two psychiatrists determined she was depressed and psychotic when she killed her baby.

The Capgras syndrome is the irrational belief that a familiar person or place has been replaced with an exact duplicate — an imposter.

Supreme Court Justice Robert Allan Hulme agreed with the psychiatrists and found her not guilty of murder.

She was sentenced in September to confinement in a mental facility.