Mishcon de Reya has been instructed by mining company BSG Resources (BSGR) in a dispute with Rio Tinto relating to rights to the Simandou iron mine in Guinea.

A letter before action sent by Mishcon on behalf of BSGR alleges that Rio played "a central role in bringing about BSGR's loss of mining rights in Simandou, Guinea" in 2011, a loss which BSGR estimates "runs to billions of US dollars".

The legal action comes after Rio last month terminated the contract of legal chief Debra Valentine amid an internal investigation into contractual arrangements with a consultant who provided advisory services on the Simandou project.

The Mishcons letter alleges that recently discovered emails, sent in May 2011, demonstrate that Rio paid a "facilitation fee/bribe" of $10.5m (£8.4m) to former banker and consultant Francois de Combret in return for assistance with negotiations with Guinea's president Alpha Conde, after which BSGR lost its mining rights in Simandou.

According to the letter, the emails were uncovered by lawyers acting for Rio on a Racketeer Influenced and Corrupt Organisations (RICO) claim made by Rio against BSGR and its former joint venture partner, Brazilian mining company Vale.

The letter states: "Rio has suppressed information regarding those emails, since they arose at least a year earlier in the context of the RICO claim. This no doubt explains why Debra Valentine has also been dismissed.

"The sacking of Debra Valentine suggests that she knew of the emails (via RICO proceedings) over a year ago, or even at the time that they were sent in 2011."

Rio conducted both an internal and external investigation into the emails in August, led by Kirkland & Ellis. On its decision to terminate Valentine's contract, the company said that she had "failed to maintain the standards expected… under our global code of conduct".

A Rio spokesperson said: "Rio Tinto acknowledges the receipt of a letter from lawyers from BSGR making a number of accusations and claims. Rio Tinto's lawyers will respond in due course. If BSGR ultimately brings a claim, Rio Tinto expects to defend itself robustly."

Mishcon partner James Libson is working on the case for BSGR.

Valentine, who joined Rio as global head of legal in January 2008, also previously worked for United Technologies Corporation, where she was deputy GC and corporate secretary.