“If the tribunal is to make decisions on laws adopted by parliament, it should be by a higher majority. Why? Because they’re lawyers, lawyers who have different points of view, both legally and ideologically,” he said.

“I find it difficult to believe how, until now, de facto three judges on the tribunal could decide the fate of a law adopted by the parliamentary majority.”

The changes make it harder for the tribunal to make decisions, by raising the bar from a simple majority to two-thirds, and by raising the quorum from nine to 13 out of 15 judges.

It also increases the lag - to up to six months - before they vet new legislation.

The ruling Law and Justice (PiS) party, which won elections in October, says reforms are needed because the judges are “cronies” of the opposition who will block its legislative programme.

But independent jurists and watchdogs, such as the Council of Europe and the European Commission, say the changes weaken a vital check on politicians’ power.

The commission vice-president Frans Timmermans, in a letter on 23 December, urged Polish authorities to put off the reforms until the debate is settled.

He said the changes risk seeing “the integrity, stability, and proper functioning of the national constitutional court undermined.”

His spokeswoman said in Brussels on Monday the commission will discuss Poland on 13 January.

The Polish opposition has reacted furiously to developments.

Grzegorz Schetyna, Poland’s former foreign minister, from the centre-right Civic Platform (PO) party, told the RadioZet broadcaster on Tuesday he has “worrying signals” from the US that it may call off a Nato summit, due in Warsaw in July, to punish PiS.

Slawomir Neumann, the PO chairman, said Duda’s decision shows that he “isn’t an independent politician.”

Mateusz Kijowski, from the Committee for the Defence of Democracy, a group which organised anti-PiS protests in recent weeks, told British daily The Guardian: “This is the end of democracy in Poland.”

Duda’s signature, the bill’s final act of adoption, comes after a previous PiS decision to remove five constitutional judges and replace them with loyalists, which was also criticised by Timmermans.

Polish media report PiS is planning, in the new year, to restrict foreign ownership of media and to extend political control over the prosecutor’s service.

Meanwhile, Neumann’s remark - that Duda isn’t independent - highlights concern the PiS party chairman, Jaroslaw Kaczynski, controls both Duda and the Polish PM, Beata Szydlo, despite not being elected to office.