Unesco said the resumption of mining at South Crofty, near Camborne, should be halted until an assessment of its impact is made.
A report published by Unesco’s World Heritage Committee stated that "mining within World Heritage properties violates the standards recognised by the World Heritage Committee and the International Council on Mining and Metals".
Cornwall Council granted permission for the owner of the South Crofty mine, Western United Mines, to develop new surface mine infrastructure adjacent on the site – as well as extend the mine’s underground working area from 510 hectares to 1,460 hectares – in March 2011.
A spokeswoman for Cornwall Council said the local authority had "consulted all the correct bodies" in relation to the application.
She added: "The Cornish Mining WHS [World Heritage Site] Office was both aware of, and involved in, pre-application discussions throughout 2009 and 2010.
"The council also consulted with English Heritage, the government’s adviser on these matters."
She added that English Heritage was satisfied that there was no adverse harm to the World Heritage Site.