Theresa May has refused to criticise Poland’s slide into authoritarianism on a visit to the country, insisting the concerns are “a matter” for its authorities.

The European Union has started a legal process that could suspend Poland’s voting rights on the Council of Ministers, warning “the entire structure of the justice system” is under threat.

The unprecedented move follows the Polish Government’s attempts to fill courts – including tribunals that will decide the validity of election results – with political appointees

Ahead of the meeting in Warsaw, Downing Street briefed that the Prime Minister would “raise her concerns with the [Polish] Prime Minister when they meet”.

But, asked about the controversy, Ms May replied: “These constitutional issues are normally, should be primarily, a matter for the individual country concerned”.

At the press conference, the Prime Minister had tough words for Vladimir Putin, saying: “The Kremlin is trying to undermine the international rules-based system. They will not succeed.”

But a difference of opinion emerged with the Polish Prime Minister, Mateusz Morawiecki, over what should happen to the one million Poles living in the UK after Brexit.

Ms May said she wanted them to stay, hailing a provisional deal with the EU – but Mr Morawiecki said he wanted them to return to Poland and its growing economy.

Brussels has cracked down after growing fears of a rising nationalist movement in Poland, operating with the tacit support of the government, led by the right-wing Law and Justice party.

For the first time, the European Commission has invoked Article 7.1, warning that a legal breakdown in one country “means breaking down the smooth functioning of the Union as a whole”.

Donald Tusk, the European Council President, protested that he had received no response to a request for a one-to-one meeting with Mr Morawiecki.

At the press conference, Ms May hailed the “relationship between the United Kingdom and Poland, based on a shared history and a shared tradition”.

And, on Article 7 specifically, she added: “Across Europe, we have collective belief in the rule of law.

“I welcome the fact that Prime Minister Morawiecki has indicated that he will be speaking with the European Commission and I hope that that will lead to a satisfactory resolution.”

Mr Morawiecki insisted that “deep reform” of the Polish justice system was needed in the light of the country’s communist past as a member of the former Soviet bloc.

And he hit back at Germany, arguing that “after the fall of communism in the German Democratic Republic, only 35 per cent of judges were positively vetted”.

Meanwhile, Ms May welcomed the signing of a “landmark” defence and security cooperation treaty between the UK and Poland, and said they would counter against Russian “disinformation” in the region.

“There could be no clearer expression of the closeness of the UK’s relationship with Poland,” she said.

Downing Street said the defence treaty was only the second such agreement the UK has signed with a European ally, after an agreement with France.