If Europeans start work on a $10bn nuclear fusion experiment before reaching an agreement with their Japanese partner, Tokyo would consider walking away from the joint project, Japan's chief negotiator warned.

The European Union says it has the financial and scientific clout to build and run a reactor in Cadarache, France, without the support of Japan. Tokyo, which says it is prepared to pay more than half the cost if the project comes to Japan, wants to host the reactor in Rokkasho-Mura.