Altice, the telecoms investor controlled by billionaire Patrick Drahi, has raised its stake in BT to 18 per cent, increasing expectations that it may eventually seek to take control of the former UK phone monopoly.

Drahi said on Tuesday that he had “engaged constructively with the board and management of BT” and looked forward to continuing that dialogue.

The government issued an immediate statement after the stake raise, saying that it was monitoring the situation “carefully” following the move. “The government is committed to levelling up the country through digital infrastructure, and will not hesitate to act if required to protect our critical national telecoms infrastructure,” it said.

The government cited the National Security & Investment Act that is due to come into force in January.

BT issued a statement saying that it would continue to operate the business “in the interest of all shareholders”.

The stake building in BT by Drahi marks a departure for the investor and entrepreneur, who is best known for taking control of undervalued telecoms assets through debt purchases then selling them off in parts.

The stake in BT, which has been built through a new UK holding company, previously stood at just over 12 per cent after Drahi surprised the British company by becoming its largest shareholder in June with a £2bn investment.

Drahi reiterated that he did not intend to make an offer for the UK telecoms group, but said that could change if circumstances do, including if a third party announces an intention to make an offer for the group or if Altice had agreement of the BT board.

Shares in BT, which are up just over a fifth so far this year, were down 3.6 per cent in early trading.

In the statement, Drahi said of BT’s management that “we continue to hold them in high regard and remain fully supportive of their strategy, principally to play the pivotal role in delivering the expansion of access to a full fibre broadband network; an investment programme which is so important to both BT and to the UK.”