The tribunal issued its ruling on Monday, public broadcaster Polish Radio's IAR news agency reported.
The bill in question foresaw that disciplinary cases of judges would be moved from the Supreme Court (SN) to the Supreme Administrative Court (NSA), among other provisions.
Critics had said that the SN's handling of disciplinary cases punished judges critical of the conservative Law and Justice (PiS) government's judicial reforms, the Reuters news agency reported.
The proposed reform was designed to allay the EU's concerns over democratic standards in Poland, such as judicial independence, and thus unblock EUR 59.8 billion in grants and loans for the country, withheld by Brussels due to these concerns, according to news outlets.
President Andrzej Duda, a Law and Justice ally, asked the Constitutional Tribunal to examine the bill and now the court has said the proposed reform is unconstitutional.
It may complicate Poland's efforts to mend relations with EU and access its funds, on the day MPs from a pro-EU coalition elected Donald Tusk as the country's new prime minister, ending Law and Justice's rule, Reuters reported.
Poland's new PM Tusk seeks to mend relations with EU
Tusk, a former top EU official, is expected to give a speech to lawmakers on Tuesday laying out his government's plans, followed by a parliamentary vote of confidence.
The president said he will swear in Tusk's Cabinet at 9 a.m. on Wednesday.
The new head of government has vowed to unblock billions of euros in EU funds for Poland.
Ursula von der Leyen, the president of the EU's executive Commission, on Monday night congratulated Tusk on his election, saying she "looked forward to working with" Poland's new prime minister.
Tusk was Poland's prime minister from 2007 to 2014 before becoming president of the European Council, a role he held until 2019.
(pm/gs)
Source: IAR, PAP, Reuters