BUDAPEST, April 9 (Reuters) - Hungary’s centre-right Tisza party leads Prime Minister Viktor Orban’s ruling Fidesz ahead of a parliamentary election scheduled for Sunday, a poll showed on Thursday.

Nationalist Orban faces the biggest challenge to ‌his rule in 16 years, although the large number of undecided voters means the outcome of the election is uncertain.

Tisza, led by former government insider Peter Magyar, had the support of 50% of decided ​voters, while 37% backed ​Fidesz, the poll by Idea Institute showed.

The survey, which had a sample size of 1,500, ‌showed 39% support for Tisza among all voters, with Fidesz backed by 30%. Some 21% of respondents said they had not decided how they would vote.

Date Agency Fidesz Tisza Our Undecided

Homela

nd

Mar Idea 30% 39% 4% 21%

29-Apr 4

Mar Iranytu 34% 41% 3% 18%

31-Apr 4

Mar 27-30 Publicus 30% 36% 4% 24%

Mar 23-28 21 Research 28% 40% 4% 26%

Centre

Mar 24-28 Zavecz 31% 39% 4% 20%

Mar 23-26 Republikon 31% 37% 4% 23%

Mar 17-20 Median 30% 46% 3% 18%

(Reporting by Anita KomuvesEditing by Gareth Jones)