The heads of Telefonica O2 Germany and KPN’s E-Plus unit have each received a letter from the Federal Network Agency that suggests they might have to give up some spectrum should the merger of the two companies go ahead, according to a report from Bloomberg.

Spectrum for 2G and 3G, wrote Iris Henseler-Unger, vice president of the agency, was granted under the condition that holders remain independent.

“As a rule, the Federal Network Agency must guard the competitive independence of owners of frequency rights in an area of scarce resources,” Henseler-Unger added. “Ultimately, the Federal Network Agency can enforce this by withdrawing the frequency assignment.”

The companies claim the O2/E-Plus transaction, which they plan to complete mid-2014, will result in cost savings and revenue synergies of between €5 billion and €5.5 billion.

Both the European Commission and the German Federal Cartel Office are due to review the proposed merger.