Vodafone has agreed to pay €18.4bn (£16.1bn) for the majority of European assets owned by the US cable giant Liberty Global.
The UK telecoms group said it will buy businesses in Germany, Hungary, Romania and the Czech Republic.
Vodafone admitted in February that it was in talks with Liberty Global about a sale.
Liberty Global owns Virgin Media, which it plans to retain, as well as its operation in Ireland.
Vodafone said that adding Liberty’s cable TV and broadband services to its mobile operations would create a “converged national challenger” to the “dominant incumbent in Germany”.
Deutsche Telekom, which is Europe’s largest telecoms provider by revenue and owns T-Mobile, has strongly objected to the move.
Deutsche Telekom’s chief executive, Timotheus Höttges, said in February that the deal was “completely unacceptable”.
He said: “I do not see that this kind of concentration in the cable market can be supported from regulatory bodies.
“I don’t believe that Germany wants to go into a situation like Eastern European markets, where TV markets are dominated by telco players.”
Vodafone already owns the largest cable business in Germany after it acquired Kabel Deutschland for €7.7bn five years ago.
Liberty Global is selling Unitymedia, which is Germany’s second largest cable business, to Vodafone.
On Wednesday, Mike Fries, chief executive of Liberty Global, said that Germany “is dominated by one provider that controls over half the broadband market”.
He said: “As a result, innovation and investment lag other countries in Europe, impacting customer service, next-generation product deployment and broadband speeds.
“Even together, Liberty Global and Vodafone, whose cable networks don’t compete or overlap, will be half the size of the incumbent operator. It’s time to alter market dynamics by unleashing greater investment and competition.”