The man Australians call "Dr. Death" came to Britain to teach a workshop on "do-it-yourself euthanasia," Sky News reported Monday.
Philip Nitschke used to be a family doctor, but 14 years ago he became an advocate for assisted suicide and now travels the world holding classes on how to end lives.
Nitschke's organization, Exit International, also sells a do-it-yourself euthanasia kit, though he stresses that no profit is made from them.
During a brief period in which assisted suicide was made legal in Australia's Northern Territory, Nitschke helped end four lives.
Although he was adamant that he does not promote euthanasia, the British government holds doubts.
"We spent 12 hours detained at Heathrow (Airport) a year ago and I spent half an hour detained as I tried to get on Eurostar in Paris two days ago while the authorities decided whether I was a fit and just person to come into your country," Nitschke told Sky News.
The Australian doctor arrived in Britain as it wrestles with the idea of assisted suicide.
After years of campaigning, last year multiple sclerosis sufferer Debbie Purdy won a landmark court case which forces the government to say when it would be likely to prosecute in assisted suicide cases.
The guidelines released last month by the Director of Public Prosecutions, Keir Starmer, said that where the victim's decision was "voluntary, clear, settled and informed," and the person assisting them acted "wholly and compassionately," prosecution was less likely. However, there are no guarantees, and assisted suicide remains illegal with a possible jail term of 14 years.