BT has sealed its £12.5billion mega-merger with mobile phone company EE, creating the UK’s biggest telecoms company.

The deal was formally completed today, having been given the green light by the Competition and Markets Authority (CMA) earlier this month. It creates a combination that will have around 35million mobile, broadband and TV customers.

The deal means that BT now holds 35% of the mobile consumer market alongside a similar share of the UK’s consumer broadband business.

It will be able to offer bundles including telecoms, TV, broadband and mobile to its customers to compete better with rivals such as Sky and Virgin Media.

Gavin Patterson, BT chief executive, said the new combination would act as “a digital champion for the UK”.

Earlier this month, however, Which? executive director Richard Lloyd said: “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.”

BT’s acquisition of EE is the latest consolidation in a sector which last year saw mobile operator O2 agree to a sale to Hong Kong conglomerate Hutchison Whampoa, the owner of rival operator Three, for £10.3bn.

Regulator Ofcom is currently carrying out its first significant review of the telecoms sector for a decade, and is considering options which include forcing BT to split off its networks business, Openreach, which provides the final mile of network connection into consumers’ homes and is used by rival operators.

Ofcom’s review is expected to be published next month.