Vodafone will give Telefonica Deutschland wholesale access to its high-speed broadband network to try to help secure approval for its acquisition of Liberty Global's cable business in Germany and eastern Europe.
Vodafone is seeking to increase competition in the German market and defuse criticism of its deal with Liberty.
It said rival Telefonica Deutschland would be able to offer super-fast services over Vodafone and Liberty's Unitymedia cable networks in Germany if the deal is approved.
The networks cover 23.7 million households.
Telefonica Deutschland has nearly a quarter of the German mobile market, second to leader Deutsche Telekom and ahead of Vodafone, according to a 2018 estimate by VATM, which represents independent telecom firms.
But it is a distant third in fixed-line broadband, with just 5.9 %.
Telefónica Deutschland's chief executive Markus Haas said the agreement would enable it to connect millions of additional households in Germany with high-speed internet.
"By adding fast cable connections, we now have access to an extensive infrastructure portfolio and can offer to even more O2 customers attractive broadband products - including internet-based TV with O2 TV - for better value for money," he said.
Vodafone, the world's second biggest mobile operator, agreed a year ago to pay $22 billion for Liberty's cable networks in Germany and eastern European markets to challenge the dominance of former monopolies such as Deutsche Telekom.
Vodafone's German rivals criticised the deal, saying it would effectively recreate a business once owned by market leader Deutsche Telekom that was broken up on the orders of the regulator when it was sold.
Vodafone and Unitymedia, Liberty's German business, operate in different parts of the country and so do not compete head to head.
But a merged entity would have a very strong position in cable and broadband, in particular to multi-family apartment blocks, leading competitors to urge regulators to step in and ensure that they also have 'last-mile' access to households.
The European Commission opened a full-scale probe into the deal in December.
Sources, however, said last month it had not raised any major concerns about the impact of the deal on Germany's cable market.
Nick Read, CEO of Vodafone Group, said the cable wholesale access agreement enabled Telefonica Deutschland to bring faster broadband speeds to their customers, further enhancing infrastructure competition across Germany.
Vodafone said it expected the EC to test the Telefonica Deutschland agreement as part of a package of remedy measures, with a final decision by July.