The stake bought by Slim, whose family controls US telco giant America Movil, has a market value in excess of £400 million (€370.5m). Slim acquired the holding through his Inbursa financial company and two other group companies.

Slim joins other high-profile investors of the company including French billionaire Patrick Drahi who acquird a 12 per cent stake in 2015 through his multinational telco Altice.

Shares in BT rose by more than 10 per cent in May after Kirkby set out her turnaround plan to more than double free cash flow over the next five years, cut more costs and potentially dispose of its global enterprise business.

A BT Group spokesperson told Reuters that the company welcomed any investor who recognised the long-term value of its business.

“We have frequent communication with all of our shareholders and meet with major investors on a regular basis,” the spokesperson said. “We look forward to engaging with Inbursa, just as we do with all investors.”