Mannesmann has rejected the $160 billion takeover offer from Vodafone Airtouch, which was launched on Sunday.
It is expected Vodafone will mount a hostile takeover bid for the German telco as early as Tuesday (UK-time).
In meeting in Dusseldorf, Germany, on Sunday, Vodafone offered Mannesmann shareholders 43.7 of its own shares in exchange for one Mannesmann share, according to Mannesmann officials.
Mannesmann officials rejected the offer overnight, calling the bid "inadequate".
Klaus Esser, Mannesmann chairman, said the telco was convinced the offer was very unattractive for its shareholders, and did not believe a consolidation with Vodafone was of strategic value.
The deal also endangers the two companies' joint ventures together, Esser said. Vodafone and Mannesmann are the major shareholders in Mannesmann Mobilfunk GmbH, Germany's largest mobile phone carrier.
Rumours of a Vodafone bid have swirled since October when Mannesmann said it would buy UK carrier Orange. Mannesmann has also become a more attractive candidate for acquisition since it announced plans in September to split off into two companies - one made of its engineering and automotive activities, the other a publicly-traded telecommunications interest - in 2001.