Ethicist Peter Singer has questioned the Medical Board of Australia’s decision to suspend euthanasia campaigner Philip Nitschke’s medical registration, and says the only way to protect vulnerable people from him is to legalise voluntary euthanasia.

In an interview with Fairfax Media, the professor of bioethics at Princeton University said it would be both ‘‘concerning’’ and ‘‘extraordinary’’ if the medical board had suspended Dr Nitschke because he believed people without a terminal illness could make a rational decision to die.

‘‘I think suicide can be rational in the absence of terminal illness and I think I could find you dozens or hundreds of philosophers who would think that ... I think if you know you are going to spend the next 20 years in prison, suicide is a rational option - not for everybody, but for some people,’’ he said, referring to the case of Nigel Brayley, a Perth man who communicated with Dr Nitschke before taking his own life while he was being investigated over his wife's death.

In July, Dr Nitschke said the board suspended his registration because his view that ‘‘people have a right to choose suicide is incompatible with his responsibility as a doctor” and because he posed “a serious risk to public health and safety that needs to be managed”.

This followed media reports that Mr Brayley, 45, was suspected to be suffering depression, had attended an Exit International workshop in February ,and communicated with Dr Nitschke before he took his own life in May.