President of French telecoms and media group Altice, Patrick Drahi smiles during the inauguration of the Altice Campus in Paris on October 9, 2018.

 French billionaire Patrick Drahi has increased his stake in British telecommunications company BT, sparking a response from the U.K. government amid fears over a full-blown takeover.

BT said its board was notified that Altice, Drahi’s telecoms group, had increased its ownership of BT to 18% from 12.1%.

Shares of BT slumped 5% Tuesday.

“The Board and management of BT Group will continue to operate the business in the interest of all shareholders and remains focused on the successful execution of its strategy and building on recent performance momentum,” BT said in a statement Tuesday.

Tracing its origins back to the founding of the world’s first public telegraph company in 1846, BT is an iconic company in U.K. corporate history. The firm was previously state-owned, but was privatized by the end of the 20th century.

In recent years, BT has struggled to convince investors of its vision to upgrade national network infrastructure and become a key player in the move to next-generation 5G mobile internet. Shares of the £16.3 billion ($21.6 billion) firm are down around 55% in the last five years.

Who is Patrick Drahi?

Drahi founded Altice in 2001. A prolific dealmaker, he made a name for himself by snapping up a slew of cable and mobile companies in Europe and the U.S.

The billionaire entrepreneur was born in Morocco but emigrated to France as a teenager. He has a net worth of $7.6 billion, according to Forbes.

He stunned the telecoms world in June when his firm paid £2.2 billion for a 12.1% stake in BT. His interest in BT has fueled concern of a full potential takeover of the company.