In Brussels' opinion, Lithuania's existing law that requires foreigners to live in the country without interruption for at least five years clearly discriminates against non-Lithuanians.
"The Lithuanian law currently does not allow citizens of other EU countries living in Lithuania to join a political party under the same conditions as Lithuanians," the EU's executive body said in a press release on Thursday.
"There is clear discrimination against non-national EU citizens, in particular those who have been resident for less than five years or whose residence has been interrupted," it said.
The Commission warned that it may refer the case to the EU Court of Justice if Lithuania's authorities fail to act within two months.
"The Commission hopes that this issue can be solved before the next elections to the Municipal Councils which take place in February of 2019 in order to allow all EU citizens living in Lithuania to participate in the democratic life in Lithuania," it said.