Tensions have risen between Poland and the bloc with the ruling of Poland's Constitutional Court earlier this month, which said the country's national laws take precedence over some EU legislation.
Polish Prime Minister Mateusz Morawiecki has accused the EU of "blackmail" in a public clash with European Commission chief Ursula von der Leyen over his country's rejection of parts of EU law.
The ferocious row, played out in the European Parliament, underlined the seriousness of the issue which both Brussels and Warsaw say threatens the cohesion of the 27-nation bloc.
Von der Leyen, speaking just before and after Morawiecki took the podium on Tuesday, warned that her Commission — tasked as guardian of the EU treaties — "will act" to rein in Poland.
She said a controversial October 7 ruling by Poland's Consitutional Court challenging the primacy of EU law was an attempt "to take an axe to the European treaties by undermining their legitimacy".
"Undermining any of these essential pillars puts our European democracy at risk. We cannot let this happen. We will not let this happen," she said.
Morawiecki, in a long speech, hit back by saying "I will not have EU politicians blackmail Poland".
READ MORE: What you need to know about the Poland-European Union rift
Dismissing MEPs' assertions that Poland had taken a step towards leaving the EU with the ruling, Morawiecki insisted his country's place was firmly in the bloc.
Instead, he argued there was a "fundamental misunderstanding" in that EU law derived from its treaties could only be applied in specified areas, and Poland's constitution was supreme in all other aspects.
He suggested the rule of law issue was being used as a "pretext" by Brussels to force Poland into line.
The duel in the parliament brought to a head tensions between the European Commission and Warsaw that have been festering for years.
The issue is bound to dominate a two-day EU summit starting Thursday where Morawiecki will again have to face plenty of critics, including EU juggernauts Germany and France.
One of the measures the Commission could use against Poland is withholding recovery cash from a pooled EU fund set up for the bloc to bounce back from the impact of the coronavirus pandemic.
Poland's plan, asking for $28 billion (24 billion euros) in grants and 12 billion euros in cheap loans, is still being weighed, with the Commission saying approval would come with strings attached.
Poland, though, has the power to play spoiler in several key EU policy areas that require unanimity among the 27 countries, among them migration and fighting climate change.
It also has the support of Hungary, another bete noire for Brussels seen to be flouting rule of law.
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