A woman has died while in custody at HMP Styal, the fourth death at the jail since the beginning of 2018.
Susan Knowles, 48, was found dead at the women's prison near Wilmslow, Cheshire, at about 01:00 BST on Friday.
Cheshire Police said officers were called to reports of a woman's sudden death and it was not being treated as suspicious.
The Prisons and Probation Ombudsman will carry out an independent investigation.
Ms Knowles' death is the fourth to be recorded at the prison in less than two years, according to government figures.
Between 2000 and 2018, there was an average of 1.3 deaths at the prison per year, although in five years - 2004, 2005, 2012, 2013 and 2015- there were no deaths recorded at all.
A Prison Service spokesperson said: "HMP Styal prisoner Susan Knowles died in prison on 10 May 2019 and our condolences are with her family and friends at this difficult time.
"As with all deaths in custody there will be an independent investigation by the Prisons and Probation Ombudsman."
Cheshire Police said: "At around 01:00 on Friday 10 May, officers were called to reports of a sudden death of a 48-year-old woman at Styal Prison, Wilmslow.
"The death is not being treated as suspicious and a case file will be prepared for the coroner."
The latest HM Inspectorate of Prisons' report, in May 2018, was positive with outcomes for women rated good in both safety and resettlement and were reasonably good for respect and purposeful activity.
Styal Prison, originally built an orphanage in the 1890s, was opened as a women's prison in 1962.
It is the only women's prison in the North West and currently operates as a resettlement facility with the aim of reducing re-offending.