Vodafone is to buy Singlepoint, the mobile phone services operation owned by entrepreneur John Caudwell, for £405m ($648m).
Vodafone is "delighted" with the deal
Singlepoint was set up in 1993, and is the largest independent mobile phone service provider in the UK.
More than 1,900 staff at the firm manage 1.9 million contract customers - 1.5 million of whom are Vodafone customers.
Vodafone UK chief Gavin Darby said the group was "delighted" with the deal.
"Together with the proposed acquisition of Project Telecom, this transaction will increase the direct access to our contract customer base in the UK to 90%."
The deal is also set push multi-millionaire owner John Caudwell - who owns 97% of the firm - into the billionaire league.
His firm, Caudwell Group, also owns the mobile retailer Phones 4U.
Singlepoint earns its money from a percentage cut on the bills of its customers.
It sells airtime onto them which is bought wholesale from Vodafone and MM02.
Its creation was spurred by government efforts to enhance competition in the sector during the 1980s and 1990s.
Mr Caudwell said: "The acquisition of Singlepoint by Vodafone UK will prove to be an extremely positive move for the company, its customers and employees."
Vodafone added the move would enable it to offer more services to "an even larger section of our customer base".
The main mobile operators are keen to hold onto customers and sell them lucrative new services.
Just last week, Vodafone bought Newark-based Project Telecom, which is the UK's largest independent service provider for business communications, for £155m.