The deal was formally completed today, after it was given the green light by the Competition and Markets Authority (CMA) earlier this month. It creates a combination that will have around 35 million mobile, broadband and TV customers.
The deal hands BT 35% of the mobile consumer market and a similar share of the UK's consumer broadband business.
BT will be able to offer bundles of 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".
But earlier this month, 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.''
Last year mobile operator O2 agreed to a sale to Hong Kong conglomerate Hutchison Whampoa, the owner of rival operator Three, for £10.3bn, further increasing the consolidation across the UK media and telecoms markets.
Aside from this deal, regulator Ofcom is carrying out its first significant review of the telecoms sector for a decade, and is considering options including a split of BT networks business Openreach.
Openreach provides the final mile of network connection into consumers' homes, and is used by rival operators.
Ofcom's Digital Communications Review is expected to be published next month.