The EU is widely expected to trigger a process that could ultimately see Poland stripped of its voting rights in Brussels, with patience wearing thin over reforms that are said to be a threat to the independence of the country’s judiciary.

If a first step in the shape of a formal warning is taken against Poland’s rightwing government on Wednesday, it will be an unprecedented act against a member state and exacerbate a growing sense of crisis over the country’s membership of the bloc.

It could also prove highly awkward for Theresa May, who will be in Warsaw for a UK-Poland summit on Thursday, at which she hopes to push forward her vision of a post-Brexit trading relationship with the rest of Europe.

The British prime minister, who will be travelling with the chancellor, Philip Hammond, and the foreign secretary, Boris Johnson, is likely to be asked which side of the argument she supports. The UK government has been ambiguous over its position, due to concerns that Brussels should not be meddling in domestic affairs.

While keen advocates of independent judicial systems, the British government also wants to build alliances as it attempts to grow support for May’s pursuit of a “deep and special” trade deal with the EU.

A row over the Polish government’s reforms to the country’s judiciary has been rolling on for two years but appears to have come to a head in the wake of the Polish senate’s decision last Friday to approve legislation giving the executive greater control of the supreme Court andnational council of the judiciary, which appoints judges.

Mateusz Morawiecki, who took over as Poland’s prime minister earlier this month, has defended the judicial changes pursued by his predecessor from the Law and Justice (PiS) party. He confirmed last week that he expected the article 7 decision on Wednesday to go against his government.

The launching of article 7, a never-before-used sanction, has been put on the agenda of a meeting on Wednesday of the European commission’s 28 commissioners, led by Jean-Claude Juncker. The commission’s vice-president, Frans Timmermans, is scheduled to address reporters in the afternoon.

If agreement is reached within the commission on Wednesday, they would offer a reasoned proposal to the member states to formally issue a warning to Poland that fundamental values were at “serious risk”.

Should at least 22 of the 28 member states then vote in favour, a warning would be issued. Poland would be given several weeks to respond ahead of potential further sanctions.

The most serious sanction possible under article 7 would be to suspend the member state of its voting rights in the council of ministers, but that would require unanimity among the member states. Hungary’s rightwing regime has insisted it would never support such a move.

Timmermans warned in the summer that the EU was perilously close to launching article 7 over the reforms, which he described as a danger to the integrity of the internal market, as well the Polish people.

Under the new laws, which are yet to be approved by the Polish president, the supreme court will be able to conduct “extraordinary reviews” of final judgments by lower courts, including those issued over the last 20 years.

An autonomous disciplinary chamber will also be created within the supreme court to be partly staffed by lay members elected by the upper house of parliament.

Supreme court judges will retire after reaching the age of 65, rather than the current 70, but the president will have the power to extend the retirement age in each individual case.

The judiciary council, which assesses candidates for the role of judge, would be elected in the main by the lower house of parliament. Until now this right was largely enjoyed by the judges themselves.

The ruling party in Poland argues that the judiciary was not properly reformed after the fall of communism and that it needs to be reorganised and purged. The opposition, Civic Platform, claim the Law and Justice’s main aim is to place loyalists at the top of judicial bodies.

Guy Verhofstadt, a former Belgian prime minister, and now leader of the liberal group in the European parliament, said: “Democracy is being dismantled in Poland. Now is the time to act. The EU must make a stand against the rule of law backsliding being implemented by the PiS government, which is totally contrary to the values of the European Union.

“This isn’t about punishing the people of Poland, who deserve better from their government.””