An approved deal would see Telefónica take over E-Plus and reduce the number of mobile operators in Germany from four to three.
Competition regulators are often concerned by such a situation, which would see Telefónica, Deutsche Telekom and Vodafone competing in the country.
The European Commission will reportedly decide on the deal by December 6 this year.
According to Reuters, analysts expect a preliminary review is likely to be extended to a five-month investigation.
The deal was first discussed in July this year and in September, KPN agreed to book a tax loss of €3.7 billion with tax authorities in Germany.
