The Serie A giants have made use of FIFA regulations to ensure they have the duo, included in a group of nine foreign-based players selected by U21 coach Marcin Dorna, with them for the start of pre-season training.
Poland will host this year's tournament, which runs from June 16 to 30, but fans will not get to witness Milik or Zielinski in action.
A letter from Napoli published by the Polish Football Association read: "In reference to the call-up of the players Arkadiusz Milik and Piotr Zielinski for the UEFA European Under-21 Championship … we would like to inform you that SSC Napoli does not authorise the release of the two players for the mentioned competition.
Poniżej pismo SSC Napoli. Brak zgody na udział Arkadiusza Milika i Piotra Zielińskiego w UEFA EURO U21. pic.twitter.com/oyR2GyY0NG— PZPN (@pzpn_pl) May 25, 2017
"We had more than 50 official matches during this season and the new one will start on 30.06.2017, when all the players will gather for the preparation of the 2017-18 season.
"As the UEFA European Under-21 Championship is not included in the FIFA International Calendar, clubs are not obliged to release their players for this kind of competition and we cannot have the players back in the club only in the second half of July."