Top US infectious disease expert Dr Anthony Fauci has urged China to release information about six labourers who fell ill after working in a mine in Yunnan province in 2012, and are now seen as a key part of efforts to find the origins of Covid-19.

The workers, aged 30 to 63, were scrubbing a copper seam clean of bat faeces in April 2012. Weeks later, they were admitted to a hospital in the provincial capital of Kunming with persistent coughs, fevers, head and chest pains and breathing difficulties. Three eventually died.

The mine is in Mojiang in southwest China, about 1,500 kilometres from Wuhan, where Covid-19 was first identified...