A 37-year-old letter penned by Lang Hancock could prove that his daughter, Gina Rinehart, has known for decades that a now-rival family has a claim to a share of lucrative mining tenements developed by her company in West Australia’s Pilbara.

Lawyers for Wright Prospecting revealed the potentially explosive 1986 letter from Hancock to his daughter on Tuesday, the second day of a trial where the company is seeking billions of dollars in past and future revenue from Hancock Prospecting’s mines in the Hamersley Ranges.