Apple is investigating its supply chain after the discovery of a disturbing cluster of leukaemia deaths among young workers at a factory in China where millions of its iPhones are made, The Mail on Sunday has learned.

At least 13 workers in their late teens and early 20's have been diagnosed with leukaemia after falling sick at the massive factory in Shenzhen since 2010. Five have died – and at ages when doctors say cases of the blood cancer are rare.

Families and a labour welfare group believe the leukaemia was caused by exposure to chemicals used to clean electrical panels and say many more workers could have been affected. They add that young workers who fall sick with leukaemia are dismissed and denied continuing medical coverage, bankrupting families as they desperately pay for treatment.