Poland’s ruling rightwing Law and Justice party is set to win a majority in the country’s parliament and secure another four-year term, according to an exit poll published at the close of voting on Sunday night.

The poll, conducted by Ipsos for private broadcaster TVN, gave Law and Justice 43.6% of the vote, which if correct would give the party 239 out of 460 seats in the Polish Sejm.

The opposition Civic Coalition, a liberal centre-right grouping, was predicted to receive 27.4%, translating to 130 seats, dampening expectations that it could remove Law and Justice from office by forging a governing coalition of opposition groups.

Observers note that snap exit polls in Poland have proved unreliable in the past, notably after European elections this year, and that even modest changes in vote share could have a significant effect on the distribution of seats, depending on how the electoral battle plays out at the constituency level.

If confirmed, however, the results would constitute a major boost for Law and Justice, which received 37.6% of the vote on a much lower turnout in the last elections held in 2015, even if its projected vote share is slightly lower than many pre-election polls suggested.

There was elation at the party’s headquarters as the exit poll was projected on a big screen, with supporters chanting the name of Jarosław Kaczyński, the party’s founder and leader, who has in effect run Poland from his party office since Law and Justice took power four years ago.

“We have reasons to be joyful – despite the powerful front that was arraigned against us, we were able to win,” said Kaczyński. “I hope that tomorrow will bring confirmation of our success. We have four years of hard work in front of us, because Poland needs to change further. And it must change for the better.”

“Today the sun shone as it rarely does in October,” said Mateusz Morawiecki, Poland’s prime minister and a Kaczyński nominee. “And I hope tomorrow it will shine even brighter. These results give us a huge public mandate.”

Also buoyant were supporters of Lewica, a coalition of leftwing parties that, according to the exit poll, received 11.9% of the vote, translating to 43 seats. The result would mark a return of leftwing parties to Polish parliamentary politics after a four-year absence, when a fragmented left failed to cross the parliamentary threshold.

“We are returning to the parliament!” Robert Biedroń, one of Lewica’s three co-leaders, told a post-election rally. “We are going back to where the Polish left has always belonged.”

The exit poll also suggested that Konfederacja, a coalition of far-right and radical nationalist groups, would also cross the 5% threshold and enter parliament with 6.4% of the vote and a projected 13 seats.

One of the leaders of Konfederacja, which receives the bulk of its support from the country’s youngest voters, is Janusz Korwin-Mikke, a veteran former MEP best known outside Poland for arguing that women are not intelligent enough to be allowed to vote.

The exit poll appears to confirm the fears of Civic Coalition and its largest party , the centre-right Civic Platform, which governed the country between 2007 and 2015.

“There will be no Budapest in Warsaw!” said Civic Platform’s leader, Grzegorz Schetyna, a reference to Kaczyński’s oft-quoted claim that his long-term intention was to emulate the so-called “illiberal democracy” pioneered by the Hungarian prime minister, Viktor Orbán.

Turnout is estimated at 61.6%, a significant rise on the just over 50% of voters who participated in 2015. If confirmed, it would be the largest turnout in parliamentary elections since the fall of communism in 1989.

• This article was amended on 14 October 2019 because Civic Coalition was predicted to receive 27.4% of the vote, not 24.1% as an earlier version said. This has been corrected.