A therapy assistant at Broadmoor Hospital has been arrested over allegations she had sex with an inmate.
The 29-year-old woman was suspended and has since resigned over the alleged incidents over the summer.
She was arrested on Friday on suspicion of being a care worker engaged in sexual activity with a mentally disordered person and later bailed.
An internal investigation has been launched at the psychiatric hospital in Crowthorne, Berkshire.
A spokeswoman for West London Mental Health NHS Trust, which Broadmoor Hospital belongs to, said in a statement: "Where there is any question that patients and staff are involved in or subject to inappropriate behaviours, prompt action is taken under the hospital's disciplinary procedure and where necessary, the hospital offers full support to the police in any enquiries or action which may follow.
"Our overarching concern is to protect the safety and rights of the vulnerable people in our care."
She added that the woman was not a nurse, but an unqualified member of staff.
Last month, Rhoda Sibanda, a former nurse at Broadmoor, was given a suspended prison sentence after admitting having a sexual relationship with a patient.