BRUSSELS -- Ursula von der Leyen, President of the European Commission, expressed her concerns on Friday after a court in Poland ruled that the country's constitution was above the European Union (EU) law.

"I am deeply concerned by yesterday's ruling of the Polish Constitutional Tribunal. I have instructed the Commission's services to analyze it thoroughly and swiftly. On this basis, we will decide on next steps," she said in a statement.

"EU law has primacy over national law, including constitutional provisions. This is what all EU Member States have signed up to as members of the European Union," she affirmed.

On Thursday, the Polish Constitutional Tribunal had ruled that Poland's constitution had primacy over EU law in Poland.

"Namely, that constitutional law is superior to other sources of law. The same has been confirmed in recent years by the constitutional courts of many Member States," Mateusz Morawiecki, Prime Minister of Poland, wrote on Facebook on Friday morning.

Morawiecki argued that double standards are applied regarding different member states.

However, EU leaders disagree.

"Today's verdict in Poland cannot remain without consequences. The primacy of EU law must be undisputed. Violating it means challenging one of the founding principles of our Union," tweeted European Parliament President David Sassoli on Thursday.

In a press statement, the European Commission said it "upholds and reaffirms the founding principles of the Union's legal order, namely that EU law has primacy over national law, including constitutional provisions."

The European Commission will proceed with its analysis of the Polish ruling, after which it "will use all the powers that we have under the Treaties to ensure" compliance with EU law.