Voluntary euthanasia advocate Dr Philip Nitschke says he is being held at a transit station in Paris, while British Customs officials decide whether to let him into the UK.
He is due to give some workshops on assisted suicide in London and interviews at the BBC in coming days.
Dr Nitschke says he is disappointed he has been stopped from boarding his train while authorities assess his status.
“They said it had come up as an alert on my passport and they needed to investigate,” he said.
“And that’s what’s going on now. They’re doing the investigation to see whether or not I can enter the country.
“This of course is a re-run of what happened a year ago, when we spent some 12 hours at Heathrow.”