Under the Polish scheme, shipmakers can pay a flat-rate tax on sales from the building and conversion of ships. The Commission said such a measure could be seen as "operating aid", which is illegal under EU rules.
"The Commission will now investigate further to determine whether its initial concerns are confirmed," the EU's competition watchdog said in a statement.
Separately, the regulator cleared a Polish scheme to aid small and medium-sized companies in the shipbuilding sector with grants and subsidies, saying the programme would encourage new investments and modernise the sector.
(Reporting by Robert-Jan Bartunek; Editing by Alissa de Carbonnel)
