The fungus responsible for so-called ash dieback disease is the same as a species of the fungus from Japan that apparently causes no harm to Manchurian ash trees in that country, they said.
The fungus that has been marching across Europe for more than 20 years, threatening many of the continent's ash trees, was first detected in Britain in the spring of this year.
It was in a small town north of Krakow in Poland two decades ago that forest rangers first realized a fungus was causing the death of their ash trees.
"My colleagues and I have come to the conclusion that this fungus cannot be native -- it's just far too aggressive," Tadeusz Kowalski of the Agricultural University of Krakow said.
Ongoing studies have suggested the fungus, Chalara fraxinea, is not a European native species and could have come from Asia, researchers said.
"Scientists working together in Japan and Germany have been looking at a fungus associated with native ash trees in Japan," Joan Webber, principal pathologist at Britain's Forestry Commission, told the BBC.
"And what they've found is that this fungus appears to be the same one causing ash dieback in Europe and now in Britain."
Because European native ash trees have not evolved resistance to it, as Japanese ash trees apparently have, they are dying in huge numbers, researchers said.