Germany stipulated that bidders for 5G mobile frequencies must invest to expand the country's mobile network and potentially allow new entrants to use their infrastructure.

The network regulator set a Jan. 25 deadline for auction submissions, saying the sale would begin in early 2019, but revised the preconditions for participation, drawing protests from the country's main network operators.

Under the revised rules, a new market entrant without its own developed network, such as United Internet, could bid for a 5G licence.

Deutsche Telekom's suit claims the new preconditions call into question its investments and discourage further development of network infrastructure, particularly in rural areas, the German paper said, citing a Deutsche Telekom spokesman.

Deutsche Telekom could not immediately be contacted by Reuters.

Germany's three main network operators - Deutsche Telekom, Vodafone and Telefonica Deutschland - have now all sued the network agency seeking to determine the legality of the new preconditions. Deutsche Telekom, in which Germany holds a 31.9 percent stake, filed a suit against the Federal Network Agency with an administrative court in Cologne, Germany, Die Welt said.

(Reporting by Edward Taylor; Editing by Andrea Shalal and Kirsten Donovan)