Carlos Slim, the Mexican telecoms billionaire, has urged archrival Telefónica of Spain to come clean about its plans in the Brazilian mobile market, the fastest-growing in South America.

The Spanish telecommunications group is part of a consortium that has agreed to take a controlling stake in Telecom Italia, owner of a leading Brazilian mobile phone operator. Telefónica already owns Vivo, Brazil’s biggest mobile phone company in a joint venture with Portugal Telecom. TI owns TIM, Vivo’s closest rival.