China has broken ground on what it says will be the world's largest hydropower project, a $170 billion feat capable of generating enough electricity each year to power Britain.

The scheme dwarfs the mighty Three Gorges Dam, currently the world's largest, and Chinese construction and engineering stocks surged after Premier Li Qiang unveiled it on the weekend.

For Beijing, the project promises clean power, jobs and a jolt of stimulus for a slowing economy. For neighbours downstream, it stirs old anxieties about water security: the Yarlung Zangbo becomes the Brahmaputra in India and Bangladesh, a lifeline for millions.