Poland’s senate has approved proposed changes to the country’s highest court that have triggered protests at home and EU warnings that they could undermine the rule of law.

The bill, which curbs the power of the constitutional court, was pushed through by rightwing senators, who hold a majority in the upper house, in the early hours of Thursday after an overnight session.

To become law, it now needs only the signature of the conservative president, Andrzej Duda, who fanned the feud over the court when he swore in four new judges this month. The lower house of parliament had approved the measures on Tuesday.

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Duda is backed by the ruling Law and Justice party (PiS), whose 58 senators voted for the changes against 28 opposed and one abstention. His spokesman tweeted: “Duda will make a decision within the deadline set by the constitution,” which is 21 days.

The opposition has denounced the law as an attempt to interfere with the constitutional court’s independence. They accuse the PiS of seeking to take control of the court and remove important checks on government power.

Poland has been plunged into a political crisis by the actions of the PiS since the party won an absolute majority in October elections. Its leader, the conservative ex-premier Jarosław Kaczyński, is neither premier nor president but is widely thought to call the party’s shots.

Kaczyński has said he wants to break up the court’s “band of cronies”, accusing it of trying to block government policies on family benefits and retirement age.

The new law says the court must approve rulings with a two-thirds majority, rather than the present simple majority, and requires 13 of the court’s 15 judges to be present for the most contentious cases, instead of the current nine. It also introduces obligatory waiting periods of three to six months between the time a request for a ruling is made and a verdict, compared with the current two weeks.

The European commission vice-president Frans Timmermans wrote to Poland’s foreign and justice ministers on Wednesday saying the law should not be “finally adopted or put into force” until all questions about its impact “have been fully and properly assessed”. Poland’s justice minister, Zbigniew Ziobro, immediately retorted that Timmermans had “been misled” by the opposition.

The office of the UN high commissioner for human rights has likewise expressed concern about the changes to the court.