Brotherly rivalry between India’s billionaire Ambani brothers is threatening to derail one of the biggest tie-ups in the history of emerging markets telecoms – between Reliance Communications and South African mobile operator MTN.

Reliance Communications, India’s second-largest mobile company headed by Anil Ambani, revealed on Friday night that Reliance Industries, controlled by Anil’s elder brother Mukesh Ambani, has claimed right of first refusal in any takeover of the cellular carrier.

The claim was conveyed in a letter sent to MTN on Thursday and to Reliance Communications on Friday, the Indian mobile operator said.

Reliance Industries’ “claim is legally and factually untenable, baseless, and misconceived”, Reliance Communications said Friday night in a statement.

Reliance Communications late last month entered a 45-day period of exclusive talks with MTN for a tie-up that would create one of the world’s largest emerging markets telecoms operators with 115m subscribers spread across Africa, India and the Middle East.

The deal is expected to take the form of a de facto reverse takeover, under which Mr Ambani will swap about two thirds of his 66 per cent stake in Reliance Communications for a 34.9 per cent stake in MTN.

While MTN will take over Reliance Communications, the share swap will mean that Mr Ambani will end up being by far the largest shareholder in the South African company, with what is expected to be a controlling stake.

However, the surprise letter from Reliance Industries means the deal is now up in the air.

Mukesh and Anil have been at loggerheads since the death in 2002 of their father, Dhirubhai Ambani, an Indian industrialist who founded Reliance Industries, a sprawling petrochemicals-to-telecoms conglomerate.

In a deal brokered by the feuding brothers’ mother in 2005, the Reliance empire was divided, with Anil taking over Reliance Communications as his flagship company and Mukesh taking over the rump Reliance Industries, whose main business is the group’s petrochemicals and oil refining business.

Since then, the brothers, who are still not on speaking terms, have fought over a range of issues, including rights to gas discovered by Reliance off India’s eastern coast.

But until Friday’s announcement by Reliance Communications, few were aware that Reliance Industries had any claims on the telecom arm.

“RIL’s actions are clearly anti-consumer, anti-investor and anti-globalisation, and against the vision, beliefs and principles of the founder of the Reliance Group, late Shri Dhirubhai Ambani,” Reliance Communications said.

“RIL’s claim is born out of mounting despair and frustration at Reliance ADA Group’s [Anil Ambani’s holding company] continuing successes.”

The group said the claim was based on an agreement in 2006, which it said was signed only by Reliance Industries officials and which was later overturned by the Bombay High Court.

It did not provide further details of the agreement. Reliance Industries was not available for comment last night.

A spokeswoman for MTN confirmed it had received a letter from Reliance Industries, which was sent to Cyril Ramaphosa, chairman of Africa’s largest mobile operator.

She added: ”This is a matter between the two brothers. As far was we are concerned, we are still engaged with Reliance Communications.”