"European Council President Donald Tusk was confirmed for a second mandate Thursday despite opposition from his native Poland, Luxembourg Premier Xavier Bettel tweeted, saying: "Habemus presidentum" of the European Council, good luck Donald!".
"There is an overwhelming support for President Tusk's re-election", Joseph Muscat told reporters.
The government argues that Mr Tusk supports the domestic opposition in Poland and has failed to protect the country's interests in the EU.
In last-minute talks Poland is trying to garner support for its counter-candidate, little-known European lawmaker Jacek Saryusz-Wolski.
German MEP Manfred Weber heads the main conservative bloc in the EU, the European People's Party (EPP).
Sources suggested that 27 of the 28 European Union states had backed Tusk in the vote.
Tusk ruled Poland between 2007 and 2014 in the centre-right Civic Platform party. If his fellow leaders back him on Thursday, he will have another 30-month term, lasting until 30 November 2019. Lithuanian President Dalia Grybauskaite said: "We don't want to become hostages of national politics inside Poland".
Malta, now chairing European Union business, is likely to seek approval of Mr Tusk by consensus. However, it appears likely that the matter will go to a vote, with Tusk being re-elected by an overwhelming majority.
"I will do everything I can to protect the Polish government against political isolation, this is for obvious reasons", he said.
Szydlo said that Tusk's support for Poland's opposition past year disqualified him as a neutral European Union official.
Party leader Jaroslaw Kaczinski said in 2012 he held Tusk "politically" accountable for the death of his twin brother, then Poland's president Lech Kaczynski, who was killed along with other 95 people in a plane crash in Russian Federation in 2010.
The plane crashed in dense fog. Polish and Russian investigations blamed pilot error for the crash.
Even Hungary - a close ally of Poland's nationalist government - made a decision to vote for Mr Tusk.