Patrick Drahi’s move to increase his control of BT puts him potentially just one step away from triggering a full takeover of the British telecoms business, although the billionaire will now have to wait six more months before he can seal the deal.

Drahi, who informed BT’s chair, Adam Crozier, that he had upped his stake to 18% after the stock market closed on Monday, can continue to increase his control, snapping up more stock, but under UK rules cannot mount a full takeover bid until 15 June.

While the founder of Altice, a telecoms and mass media company, has so far positioned himself as a benevolent investor, his decision to buy more stock on the first trading day after the expiration of a no-bid clause triggered when he made the surprise move to become BT’s biggest shareholder in June, sends a clear reminder of his activist intentions.

One City source said the stake-building approach marked a strategy of “creeping control” without having to pay the premium that would come with a bid offer. Drahi is understood not to have outright economic control of all of the 18% he now holds, but he does have the voting rights.

Drahi’s Altice UK is understood not to have acquired any of the 12.06% stake held by Deutsche Telekom (DT). Tim Höttges, the chief executive of BT’s second-largest shareholder, has said he is “entertaining all options” and referred to DT as a “kingmaker” in regards to the future of the UK company.

A deal between Drahi and DT, which has held its stake in a passive capacity as a legacy of BT’s takeover of the mobile operator EE in 2015, would already pass the 30% threshold at which a takeover offer must be tabled.

Until mid-June 2022 Drahi, who has not as yet requested a seat on the BT board, can only make a move for majority control with the agreement of BT’s board or if a third party emerges with an offer.

BT confirmed that Crozier, the former chief executive of ITV and Royal Mail who officially took over as the chairman at BT on 1 December, and its chief executive, Philip Jansen, met with the culture secretary, Nadine Dorries, on Monday. The meeting, which was understood to be about BT’s role in the national rollout of 5G and broadband, was prior to Drahi snapping up the extra 6% stake.

On Tuesday, the government said it would “not hesitate to act” to protect BT, which is considered critical to the UK’s national communications infrastructure, from foreign takeover if it was deemed necessary.

BT has strengthened its defences by hiring advisers at Robey Warshaw as Drahi looks to cash in on the apparently undervalued telecoms company, which is trading at 65% below 2015 levels.

Drahi’s move followed BT’s £15bn commitment to the rollout of full-fibre broadband to 25m homes by 2026, having gained valuable regulatory and financial incentives guaranteeing significant returns on its huge investment over the long term.

BT’s share price fell more than 5% on the news, making it one of the biggest fallers in the FTSE 100, as investors’ hopes of a takeover for Christmas failed to materialise.