BT has entered exclusive negotiations to buy EE, the UK's leading mobile operator EE for £12.5 billion, gaining 24.5 million customers in the process.
The period of exclusivitiy is set to last for "several weeks", giving BT chance to carry out the necessary due diligence.
"The proposed acquisition would enable BT to accelerate its existing mobility strategy," the telecoms giant said in a statement.
EE is the UK's largest mobile network group, with 33.8 percent of the UK mobile market, according to Citigroup. If BT completes the acquisition then it will be able to offer fixed-line phones, broadband, mobile and TV.
BT announced at the end of November that it was in talks to buy either EE, jointly owned by Deutsche Telekom and Orange, or O2 which is owned by Spanish firm Telefonica.
In 2002, BT spun off its own mobile network, BT Cellnet, now O2. It went on to be acquired by Spain's Telefonica for £17.7 billion in 2005.
Dominic Baliszewski, telecoms expert at broadbandchoices.co.uk, said: “BT has gone full circle with the acquisition of mobile network EE, having only left the mobile space 12 years ago when it sold BT Cellnet.
"The trend for providers to offer the full suite of home comms services (broadband, TV, mobile, landline) is showing no sign of slowing, but UK customers have not yet embraced quad-play as enthusiastically as in other European countries, such as Spain, where quad-play is hugely successful. UK customers largely view a mobile phone as a personal purchase rather than part of their household spend so a fundamental shift in perception is needed to make quad-play the norm.
“That being said, quad-play does have the potential to save customers a great deal of money and BT in particular has the infrastructure at its disposal to offer some truly compelling pricing alongside robust coverage (both from EE's best-in-market UK mobile coverage and BT’s 5 million Wi-Fi hotspots). Throw in some tasty freebies in a similar vein to the ‘free BT Sport’ offer of the previous 12 months and you have a tempting proposition."