The judgment of the Polish Constitutional Tribunal, which states that some EU provisions are inconsistent with the country's constitution, is now officially legally binding.
This comes after the ruling was published on Tuesday in the official court journal.
The Polish decision is perceived in EU circles as a possible first step towards Poland's exit from the EU. The Tribunal rules that important parts of several EU treaties are inconsistent with the Polish constitution.
The EU tried to intervene against the reforms of the national judiciary, the conservative ruling party PiS. The EU believes that the reforms undermine the independence and democracy of the judiciary.
Tens of thousands of Polish citizens gathered on Sunday for demonstrations to support Poland's membership of the EU. Surveys show that EU membership continues to be very popular with the population.
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