BUDAPEST, April 8 (Reuters) - Hungary's opposition party Tisza is on track to win a two-thirds parliamentary majority in Sunday's election, allowing it to amend the constitution and key laws needed to unlock EU funds, a projection from polling agency Median showed on Wednesday.
Veteran nationalist Prime Minister Viktor Orban and his Fidesz party are facing the biggest challenge to their rule in 16 years as the centre-right Tisza party, led by Peter Magyar, is leading in independent polls.
Tisza is seen winning between 138 and 142 seats in the 199-member parliament, according to an estimate based on an analysis of Median's five most recent opinion polls conducted in late February and March.
Fidesz is expected to secure between 49 and 55 seats while the far-right Our Homeland (Mi Hazank) party is estimated to win five or six seats, the projection showed.
In Hungary's parliament, a party needs 133 seats to obtain a supermajority required to amend the constitution and key laws.
Fidesz has held a two-thirds majority throughout most of its rule since 2010, and it used that power to approve a new constitution and to pass and amend several cardinal laws, including the electoral law.
Pollster Median, which has one of the strongest track records of accurate forecasting in Hungary, said its five surveys had a sample size of 5,000 in total and were conducted by three separate call centres.
It correctly predicted Orban's landslide victory in the last election four years ago, though it slightly overstated support for the opposition.
While most polls have shown a Tisza lead, Fidesz points to surveys that still show Orban's party is on course to victory. Fidesz's opponents say these have mainly been conducted by institutes with financial or personal ties to the ruling party.
(Reporting by Anita Komuves; Editing by Jason Hovet and Gareth Jones)
