UPDATE 1-Rule of law clash with Poland set to overshadow EU summit

(Adds details and background)

By Jan Strupczewski and Gabriela Baczynska

BRUSSELS, Oct 20 (Reuters) - Poland's clash with Brussels over a court ruling that questioned the supremacy of European Union law has been added to the agenda of a leaders' summit this week at the request of several countries that see it as a serious crisis for the bloc.

"We will ... touch upon recent developments related to the Rule of Law during our working session," European Council President Charles Michel said in a letter to leaders of the 27 member states ahead of the Oct. 20-21 summit he will chair.

A senior EU diplomat said 12 countries had asked for the matter to be discussed at the meeting in Brussels, in addition to the spike in energy prices and the COVID-19 pandemic.

"There is a concern that this could lead to a situation in which our agenda is thrown into disarray," another senior EU diplomat said, adding that the stand-off with Poland could be discussed early in the summit to "clear the air".

Poland's relations with the EU went from bad to worse on Oct 7 when its Constitutional Tribunal ruled that elements of EU law were incompatible with the Polish constitution.

Brussels has long accused Poland's nationalist government of undermining its judiciary's independence, but the ruling calls into question the legal pillar on which the EU stands and has raised fears that Poland could eventually exit the bloc.

Poland says it has no plans for a "Polexit" and, unlike Britain before its Brexit referendum in 2016, popular support for EU membership remains high in the eastern European country.

European Commission President Ursula von der Leyen, branding the Polish tribunal's ruling as "a direct challenge to the unity of the European legal order", on Tuesday laid out three options for a response to Warsaw, ranging from legal action to a cut in funding a suspension of its EU voting rights.

Separately, the president of the EU's parliament said on Wednesday he had asked his legal department to start preparing a lawsuit against the bloc's executive, the European Commission, for its failure to apply new rules that would halt funding from the EU budget to Poland.

He wants the Commission to apply a so-called Conditionality Regulation, which allows the for the suspension of payments to member states where the rule of law is under threat.

"The European Union is a community built on the principles of democracy and the rule of law. If these are under threat in a member state, the EU must act to protect them," he said in a statement. (Reporting by Jan Strupczewski, John Chalmers, Gabriela Baczynska and Philip Blenkinsop Writing by John Chalmers)