The telecoms industry has raised concerns over the approval of the merger between EE and BT by the Competition and Markets Authority (CMA).
After being given the green light, the new company will have a combined customer base of around 35 million mobile, broadband and TV customers.
Despite the criticisms, John Wotton, who chaired the CMA inquiry into the deal, said the evidence "does not show that this merger is likely to cause significant harm to competition or the interests of consumers".
Broadband and mobile provider TalkTalk is one such industry player that is not happy with the new deal.
"We are disappointed, although not surprised, that the CMA has waived through the BT/EE merger, even though the new entity will be even more dominant than it was before privatisation 30 years ago," it said.
Which? executive director, Richard Lloyd, added: "Fewer players in a market is rarely a good thing, but now this deal has been approved both companies must urgently address their abysmal customer service record.
"The regulator will need to keep a very close eye on this to ensure consumers really do benefit from the deal and be prepared to step in if not."
The deal hands BT 35 per cent of the mobile consumer market and a similar share of the UK's consumer broadband business.
Gavin Patterson, BT chief executive, said: "The combined BT and EE will be a digital champion for the UK, providing high levels of investment and driving innovation in a highly competitive market."
The merger follows the purchase of 02 by industry rivals 3, which is owned by the Hong Kong-based Hutchison Whampoa, for £10.25bn last year.The regulator, Ofcom, is set to carry out its first significant review of the telecoms sector for a decade, and is considering options including a split of BT networks business Openreach from its parent company.
Openreach provides the final mile of network connection into consumers' homes, and is used by rival operators.