The Prison Service said last night that 13 cases of E.Coli 0157 had been confirmed among prisoners.
A spokesman said it was the first time that the Prison Service had experienced an outbreak of the dangerous pathogen, which was responsible for the deaths of 21 people in Lanarkshire, Scotland, in 1996.
The problem first emerged last Tuesday, when 25 prisoners reported stomach pains and sickness to staff.
In three of the most worrying cases, the sick inmates have been transferred to Leicester Royal Infirmary, where they are being kept under observation. None of the three are believed to be seriously ill. Eight others are being cared for in the prison's health-care unit.
The Prison Service spokesman said that the prison governor was working with local authority environmental health inspectors to "monitor and manage" the outbreak.
The health inspectors have given the prison kitchen the all-clear and were still searching this weekend for the cause of the problem.
The Prison Service spokesman said: "No new prisoners have become ill since last Tuesday."
The incident has been a source of tension between the jail's management and prison officers who claim they have been kept in the dark about the cause of the illnesses.
Leicester Prison is a local jail in a residential area, catering for both high security and low security prisoners who are awaiting trial, sentence or prison allocation following conviction.Reuse content