"This Commission will take its duty seriously even if, politically, this might be extremely difficult," he said.

His mention of "every instrument" referred to article 7 of the EU treaty, under which a member state can lose its vote in the Council if it is deemed to be in "serious breach" of EU values.

The unprecedented move would need Council unanimity, but Timmermans said "a very broad majority" of EU states were already unhappy with the Polish government.

He spoke after Poland rejected as "groundless" the Commission's complaint that the ruling Law and Justice (PiS) party was trying to seize control of Polish courts.

The Commission is still analysing Poland's 12-page long statement of reasoning.

Timmermans said on Thursday: "I cannot tell you [MEPs] today what will be the next step". But he refuted Poland's main arguments - that the Commission had no jurisdiction on the issue and that the PiS reforms were normal.

"I honestly would beg to differ … there's no other member state where the minister of justice can arbitrarily fire judges without any conditions," he said.

"If you go back to a situation where the rulings of a court are determined by a call from party central then we have a problem", he said.

He noted that Poland had also ignored an EU court order to stop logging in Bialowierza, a primeval forest, and an EU Council vote to take in asylum seekers.

He said the Polish case was of central importance.

"It is who we are and what defines us as Europeans - a rule of law-based democracy," he said.

Getting personal

Marek Jurek, a right-wing Polish MEP, defended PiS in the EU parliament hearing.

He said Timmermans should not make threats because only EU states had a mandate to decide on Poland.

Jurek said all that the Commission could do was to "submit a request [for sanctions] to the Council and wait patiently for its decision".

"The European Commission has to respect the fact that there is no unanimity among member states when it comes to accusations against Poland," the MEP said.

He accused Timmermans of siding with the Polish opposition and of "Marxism, which treated laws as an instrument in the hands of power".

Other Polish MEPs heckled Timmermans in Thursday's unruly hearing, yelling out "It's not true!" as he spoke.

Udo Voigt, a far-right German MEP, accused him of attacking Poland under false pretences.

"You [Timmermans] don't really give a toss about the [Bialowierza] forest being cut down", he said.

Dangerous, populist

The main groups in the EU assembly were on the Commission's side.

Roberta Metsola, a Maltese MEP from the centre-right EPP group, said what PiS was doing was "dangerous, it is populist, and, crucially, it is legally flawed".

A Swedish liberal deputy, Cecilia Wikstroem, said: "Where they [PiS] have failed, we will prevail".

PiS' actions amounted to a "systematic withdrawal from the rule of law", Judith Sargentini, a Dutch Green MEP, said.

Timmermans said he had invited Polish ministers to discuss the issues with him in Brussels. They did not reply to him, but they told Polish media they would not come.

He said PiS had demanded "respect", but that "this has not really been reciprocated, may I say, in the least".

"I don't care if they want to continue to attack me personally and call me stupid and incompetent and so on," he said.

Divided Europe

He said PiS was wrong to claim that the Commission was trying to infringe on Polish sovereignty.

"Poland today is more sovereign, more free, and more secure than for centuries in the past. It has been perhaps 1,000 years since Polish people have been as free to decide about their own destiny," Timmermans said, referring to Poland's EU membership and to its history of German and Russian partitions.

With Thursday's hearing setting a divisive note after the EU institutions' summer break, the commissioner said European unity had geopolitical value.

"Whoever is in charge in Russia today would like to see a divided Europe", he said.