A murder investigation has been launched after a 73-year-old prisoner - serving a nine-month sentence for voyeurism - was found dead in his cell.
Alan Goode, a serial peeping tom, was jailed in October after he was found lying on a cubicle floor in the women's toilets of a Leicester shopping centre.
An ambulance attended after staff found him in his cell at Lincoln Prison at around 5:45am this morning but he was pronounced dead at the scene, the Prison Service said.
A 35-year-old male inmate - understood to have been sharing a cell with Goode - has been arrested in connection with the incident, Lincolnshire Police said, as it opened a murder investigation.
Goode admitted voyeurism and breaching a sexual offences prevention order which was imposed in 2009 and banned him from loitering near or entering female toilets for five years, according to reports.
While jailing Goode for nine months, Judge Robert Brown at Leicester Crown Court also extended the order - banning him from ladies' lavatories for the rest of his life.
He had previously been jailed for nine months for again flouting the banning order, by spending 90 minutes in the toilets at John Lewis in Leicester.
Goode was also given suspended jail sentences for two earlier incidents of voyeurism at bars in Leicester city centre.
HMP Lincoln is a category B prison, which can hold up to 720 adult remand and convicted prisoners, including life-sentenced inmates.
The Victorian jail serves the courts of Lincolnshire, Nottinghamshire and Humberside.
In December last year, prison inspectors p roduced a damning report into safety standards at the overcrowded jail.
Her Majesty's Inspectorate of Prisons (HMIP) said it found serious failings at the jail, which at the time was holding 50% more prisoners than it was certified to hold.
Juliet Lyon, director of the charity Prison Reform Trust, said: "Investigations by police, Prisons Ombudsman and the Prison Service must examine how a frail pensioner's troubled life came to such a brutal end in Lincoln jail."