Earlier BSGR said it had sent a "pre-action" letter to Rio Tinto, alleging Rio Tinto contributed to the loss of its mining rights in Simandou, the latest twist in two decades of attempts to exploit the world's largest untapped iron ore reserves.
Rio Tinto faces a separate legal action from a former senior executive, one of two sacked in November following an inquiry into $10.5 million (8.3 million pounds) in payments to a consultant who helped Rio win rights to develop iron ore lode in the Simandou mountains.
BSGR said that unless Rio Tinto gave a satisfactory response to allegations set out in the letter by Jan. 3, 2017, and made "proposals to compensate BSGR for the extensive loss and damage suffered", proceedings would be issued without further notice.
Rio Tinto acknowledged receipt of the letter in an emailed statement and said its lawyers would respond in due course.
"If BSGR ultimately brings a claim, Rio Tinto expects to defend itself robustly," it said.
Rio Tinto acquired an exploration licence to develop blocks in the Simandou mountains in 1997 but following years of disputes over the rights and the difficulty of financing complex infrastructure plans, Rio said in October it was selling its stake in the project to Chinalco 3668.HK.
In November it announced the suspension and then sacking of two top executives in connection with the Simandou project.
Reporting by Barbara Lewis; Editing by Alexander Smith