The decision by Carlos Slim to buy a £400m (€473m) stake comes weeks after the company unveiled a major shake-up.
Share in BT jumped by 10% last month after BT's newly appointed CEO Alison Kirby outlined plans to double cash flow over the next five years and implement an aggressive strategy to cut £3 billion in costs.
Carlos Slim who made his fortune in telecoms in Latin American also has interests in energy, banking and construction. He joins the French telecoms billionaire, Patrick Drahi and the former Germany monopoly Deutsche Telekom as major BT shareholders.
A spokesman for Mr Slim's Grupo Corso told the Financial Times that the move was a "financial investment, like many the group makes". New Street Research analyst James Ratzer told the Telegraph that Mr Slim's investment would be viewed as a "vote of confidence" in BT's leadership and its new strategy.
A BT spokesman told the Telegraph: "We welcome any investor who recognises the long-term value of our business.
"We have frequent communication with all of our shareholders and meet with major investors on a regular basis. We look forward to engaging with Inbursa, just as we do with all investors."
James Bamford from Enders Analysis told the Telegraph that the Mexican tycoon may have been attracted to buy a stake in BT because of potential payouts on offer from the company.
BT's ability to pay large dividends has been restrained for many years by the roll out of fibre broadband in the UK. But this project is due to end in 2026 and many investors believe that BT will then be able to start paying out more cash.
