Correction Appended

BERLIN, July 9 — Already weakened by coalition squabbles and disagreements with the European Union, President Lech Kaczynski of Poland announced over the weekend that on Monday he would appoint his twin brother and longtime political partner, Jaroslaw, to succeed Prime Minister Kazimierz Marcinkiewicz, according to the Polish news agency PAP.

The resignation of Mr. Marcinkiewicz, considered one of the most popular politicians in Poland and one of the more moderate voices in the national-conservative coalition government, was confirmed Saturday. Neither the government nor the president's office would give an official reason for his resignation. The Kaczynskis have been trying to increase their grip on the fragile governing coalition as local elections approach in November.

Mr. Marcinkiewicz, 46, a physicist, was a surprise appointee last October. When the Law and Justice Party won parliamentary elections the preceding month, Jaroslaw Kaczynski, who founded the party with his twin, was expected to become prime minister. But after Lech Kaczynski was elected president, he said it would send the wrong signal to the outside world if he and his twin held the two most powerful jobs in Polish politics.

Indeed, some analysts said they feared the appointment would further weaken Poland's role in Europe.