Slim's company, Control Empresarial de Capitales, crossed the regulatory threshold requiring it to report its involvement on June 6, BT explained in a presentation on Wednesday.

The family investment firm of Mexican billionaire Carlos Slim has revealed a 3.2% stake in British telecommunications company BT, whose new CEO has reversed falls in its shares and increased the dividend in the last month.

Slim's company, Control Empresarial de Capitales, crossed the regulatory threshold requiring it to report its involvement on June 6, BT explained in a presentation on Wednesday.

Slim, 84, is the 17th richest person in the world, according to the Bloomberg Billionaires Index. The Mexican magnate made most of that fortune with América Móvil, the largest mobile telephone operator in Latin America.

Aside from Slim's family investment company, América Móvil is also the largest shareholder in Telekom Austria AG and owns a stake in Dutch company Royal KPN NV.

BT shares rose 1.1% in Wednesday's London session.