The family of a patient killed by an out-of-hours German locum has accused the medic of trying to silence them through a Europe-wide injunction.
Stuart and Rory Gray are trying to bring to Britain Dr Daniel Ubani, who fatally administered 10 times the normal dose of diamorphine to their father David, 70, at his home in Manea, Cambridgeshire.
The brothers, who recently barracked him at a medical conference in Germany, claim the doctor is trying to gag them using European human rights laws by claiming their campaign is stopping his right to practise.
Stuart Gray, himself a doctor, told the Daily Mail: "I consider this a grave threat to free speech and we will fight it in every way possible."
Ubani has reportedly submitted papers to a German court calling for the Gray brothers to be banned from talking about the death.
Rory Gray, 45, said: "Ubani is attempting to censor what we say, not only to the German, but also to the British media. I am astonished that a German court is allowed, or even being requested, to censor the British media."
Lawyers for the Gray brothers are expected to argue at a court in southern Germany that speaking about what happened to their father is a fundamental human right.
Following Mr Gray's death in 2008, Dr Ubani admitted being exhausted after getting only a couple of hours sleep before starting his shift in the UK, and said he was confused about the difference between drugs used here and in Germany. His poor English meant he was refused work by the NHS in West Yorkshire but was later accepted in Cornwall.
A UK coroner recorded a verdict of unlawful killing in the inquest into the death of Mr Gray, also accusing Dr Ubani, who was struck off by the UK's General Medical Council in June, of gross negligence.
He was given a suspended sentence in Germany for death by negligence but is still able to practise there.