Poland's president on Sunday pledged full support for Ukraine's EU membership bid, saying those who "shed their blood" for Europe must be respected.
Andrzej Duda's comments contrasted with the reservations of EU heavyweights France and Germany, who have warned Ukraine's application would not be fast-tracked despite Russia's invasion.
France's Europe minister on Sunday said it could take "15 or 20 years" before Ukraine joined the bloc.
Speaking at a joint news conference with Ukrainian President Volodymyr Zelensky, Duda said those who "shed their blood" to belong to Europe must be respected, "even if the situation is complicated, even if there are doubts".
"I have no doubt that the European Union will make such a gesture," he added, saying the European Council's decision on Ukraine's candidate status on June 24 would be "extremely important, above all psychologically and politically".
Addressing the Ukrainian parliament in Kyiv, Duda said he would not let up his efforts as long as Ukraine remained outside the bloc and that a successful membership bid would be thanks to Warsaw.
Poland has welcomed by far the highest number of Ukrainian refugees and has been Kyiv's main supporter within the European Union.
In contrast, Ukrainian Foreign Minister Dmytro Kuleba on Thursday denounced the "second-class treatment" Kyiv has received from some EU countries.
- End to 'business as usual' -
Duda also said "business as usual" with Russia was impossible after the alleged mass killings of Ukrainian civilians and war crimes blamed on Moscow's forces.
Hundreds of civilian bodies have been found in towns near Kyiv previously occupied by Russian troops such as Bucha and Borodianka.
The southeastern port city of Mariupol lies in ruins after a weeks-long Russian siege that Ukrainian authorities say killed at least 20,000 civilians.
"After Bucha, Borodianka, Mariupol, there cannot be 'business as usual' with Russia," Duda told the Ukrainian parliament, in the first such speech by a foreign head of state since war began on February 24.
"An honest world cannot return to business as usual while forgetting the crimes, the aggression, the fundamental rights that have been trampled on," the Polish head of state added.
Duda expressed regret that some European countries have asked Ukraine to "accept certain demands" from Russian President Vladimir Putin for their economic interests or political ambitions.
Speaking in the presence of Zelensky and interrupted by several standing ovations, he warned that the most minor concession of Ukrainian territory or sovereignty would be a "big blow" for Ukraine and the West.
"Only Ukraine has the right to decide its future... There cannot be negotiations or decisions taken behind Ukraine's back," Duda said, lauding the country for defending Europe against a "barbarian invasion and the new Russian imperialism".
Zelensky thanked Duda for "his visit, his support and his true friendship" in an Instagram post.