In a letter seen by the Financial Times, the Swiss Office of the Attorney-General said its inquiry was not directed against the Anglo-Australian mining group.

“Currently, the investigation is directed neither against your client nor against any of client’s employees,” the OAG letter reads.

The OAG is conducting a criminal investigation into a frozen Swiss bank account that the Mongolian Anti-Corruption Authority (MACA) says could have been used to pay a bribe

Rio controls Oyu Tolgoi through a Canadian-listed subsidiary called Turquoise Hill.

Earlier this month, Turquoise Hill revealed earlier this month it had received a request for information from the MACA in connection with the “possible abuse of power by authorised officials” during the negotiation of the 2009 Oyu Tolgoi investment agreement.

At the time the deal was signed nine years ago Rio owned a 10 per cent stake in Turquoise Hill, which was then called Ivanhoe Mines. Rio later raised its interest to 51 per cent and now operates the mine.

The agreement, which has attracted plenty of controversy, paved the way for development of the mine and set out the terms of Mongolia’s part ownership and taxes to be paid.

A row over the agreement in 2013 led to a two-year delay in the underground expansion project.

More recently, the Mongolian government has hit Turquoise Hill with a $155m tax claim and ended a deal that allow Rio to source electricity for the mine from across the border in China.

An underground expansion of the mine, situated in Mongolia’s Gobi Desert, is one of Rio’s most important growth assets.

The company is ploughing $5bn into the project aimed at lifting output to 560,000 tonnes of a copper a year between 2025 and 2030.

News that Rio is not being investigated by Swiss prosecutors will be a relief for the London-listed miner, which is facing civil US fraud charges in connection with a bungled African coal deal.

Regulators in the UK and US are also examining a payment Rio made to a French consultant who helped the company secure the rights to large iron ore deposit in Guinea.