On the eve of new Polish measures that could mean dozens of senior judges losing their jobs, the European Union has launched a legal case against the country's right-wing government.
The EU's executive has already accused the Polish authorities of bringing in laws that "interfere significantly" with the judiciary.
Now it has given Warsaw a one-month deadline to answer its concerns.
Under the laws, up to 40% of Supreme Court judges could be forced out.
The first president of the court, Malgorzata Gersdorf, could be among those pushed out before her six-year term ends, by a rule that brings down the retirement age from 70 to 65, the EU warns.
Why are the reforms so controversial?
Poland insists its reforms, which have already come into effect, are aimed at updating an inefficient system and replacing judges who date back to a communist era that collapsed in 1989.
The EU launched a legal case last December, arguing that the independence of the Supreme Court and Constitutional Court have been undermined by the package of reforms.
Now it has increased its pressure on Warsaw over a measure that comes into force on 3 July. All judges over 65 have been given until that date to ask President Andrzej Duda to prolong their mandates.
The president has the power to grant an extension. However, the EU complains that the judge has no way of seeking a review and the head of state does not have to explain any reasons for turning such requests down.
The EU's executive, the European Commission, has given Poland a month to respond. It normally gives two months. Foreign Minister Jacek Czaputowicz said the law was binding and "for the time being our stance is that we are right".
Why has the EU acted now?
By Adam Easton, BBC News Warsaw
The European Commission had to act quickly, ahead of the president's deadline on Tuesday to decide whether the judges can stay.
Many in Brussels, as well as international legal and human rights groups, are concerned that the ruling Law and Justice (PiS) party is trying to politicise the judiciary and erode the separation of powers.
Since taking office in November 2015, Law and Justice has passed legislation that has already given it control of the Constitutional Court, which can veto legislation, and the National Council of the Judiciary - the body that nominates judges in Poland.
Justice Minister Zbigniew Ziobro has used another law to change more than 160 ordinary court presidents or their deputies - that's almost 20%.
The Commission has been in talks with Warsaw trying to resolve the issue since January 2016. Earlier this year Poland's government made amendments to some of the laws, but Brussels deemed them insufficient. The problem with those discussions is that Warsaw can and has called them political. Brussels is overstepping its boundaries, it says.
On the other hand, legal action against Poland, which could eventually lead to the imposition of daily fines if Poland is deemed to be in breach of EU law, could be more effective in giving Warsaw pause for thought and also easier for it to accept.