Right to Die?, which will show the moment when 59-year-old motor neurone disease sufferer Craig Ewert died, is believed to be the first time a person's death via an assisted suicide has been shown on British television.
Documentary makers were allowed to film the final moments of Ewert's life at controversial Swiss clinic Dignitas.
Right to Die? shows Ewert drinking a fatal dose of barbiturates, prescribed by a local doctor.
Shortly before closing his eyes, he is heard to say: “Thank you”. He died 30 minutes later.
Ewert's widow, Mary, has defended the decision to allow the cameras inside the clinic, which helps people to commit suicide.
Assisted suicide is legal in Switzerland with certain conditions, but it is illegal in Britain.
The programme will air at 9pm tonight.