BUDAPEST—Poland has no desire to leave the European Union, Prime Minister Mateusz Morawiecki said on Tuesday, accusing opposition politicians of spreading what he said were lies about a possible “Polexit.” His comments came after the Polish Constitutional Tribunal ruled last week that parts of EU law were incompatible with the country’s constitution, undermining a key tenet of European integration and fuelling talk that Poland could one day quit a Union that has helped drive its economic development. The ruling was published in the official gazette on Tuesday, making it legally binding. More than 100,000 Poles took part in rallies on Sunday in support of EU membership. “Our dear opposition is trying to insinuate that we want to weaken the union by leaving the EU,” Morawiecki said during a summit of the four central European “Visegrad” nations in the Hungarian capital Budapest. “This is obviously not just fake news, it is …