BUDAPEST (Reuters) -Hungary's main opposition party Tisza has a 15-point lead over Prime Minister Viktor Orban's Fidesz among decided voters, widening the gap between them ahead of next year's election, a survey by pollster Median showed on Wednesday.

The centre-right Tisza party, led by Peter Magyar, poses the biggest political challenge to nationalist Orban's 15-year rule while his government is also facing economic difficulties.

Hungary's economic output was unchanged in the first quarter from a year earlier, while inflation was among the highest in the European Union. The threat of steep U.S. tariffs on EU imports also looms large over Hungary's recovery prospects.

Magyar's party, which was only launched last year, has the support of 51% voters who say they have decided whom to back, while 36% of respondents said they would vote for Fidesz according to the poll, published by local news weekly HVG.

The survey showed that Tisza was particularly popular among younger people, with 58% support among under-40s, while a majority of those aged over 50 backed Fidesz.

A majority of those polled said they expected Tisza to win the next election.

"This is noteworthy because, in the past two decades, public opinion polls preceding elections almost invariably predicted a Fidesz victory, thus reliably forecasting the ultimate outcome," Median researchers wrote in the article.

Spokespeople for the government and Fidesz were not immediately available for comment.

Tisza, named after Hungary's second biggest river and founded by Magyar, a former government insider, in February 2024, has led Orban's party in most opinion polls for months.

Orban, keen to lure back disaffected voters, included steep tax cuts for families in a 2026 election year budget approved by parliament on Tuesday.

The parliamentary election is set to take place early next year, though no date has yet been set.

Ahead of the 2022 parliamentary election most pollsters predicted a close race as they overestimated support for the opposition parties. Among the pollsters, Median produced one of the most accurate predictions of the 2022 race in a poll conducted shortly before the election.

(Reporting by Anita KomuvesEditing by Gareth Jones)