Last year's election which got Hungarian Prime Minister Viktor Orban a third term in power with a landslide parliamentary majority was marred by "major malpractice", a rights group said Saturday.
Authored by Unhack Democracy Europe and published on the OpenDemocracy website, the report detailed widespread alleged vote-rigging and fraud in the April 2018 vote.
Incidents it described as "major malpractice" include transporting voters from neighbouring countries like Ukraine, bribery and intimidation especially in villages, tampering with postal votes, missing ballots and election software malfunctions.
The findings raise "serious" concerns about the integrity of the European parliamentary elections taking place in Hungary on May 26, it said.
"The same election office that ran a tainted election last year will be once again tallying the results," said an expert cited in the report.
The government's press office has not yet responded to requests for comment.
Polls indicate Orban's ruling nationalist Fidesz party is likely to win over 50 percent of votes cast by Hungary's eight-million-strong electorate, and around 14 of the EU member state's 21-seat allocation in the European Parliament.
Fidesz won almost half of the general election vote in 2018, granting it two-thirds of seats in the 199-seat assembly in Budapest, which allows it sweeping powers.
Orban's critics accuse him of using those powers to systematically dismantle democratic checks and balances like judicial and media independence since his first landslide win in 2010.
Election monitors from the OSCE had said the 2018 ballot was "efficiently administered" but held in an "adverse climate" that helped Fidesz win.
In 2014, the OSCE called the Hungarian election system "free but not fair" after Orban rewrote the election rules following his landslide win in 2010.
Electoral districts were redrawn in ways that favoured Fidesz, while postal votes were granted to ethnic Magyars in neighbouring countries but not to emigre Hungarians who are less likely to vote for Orban's party.
A second round of voting was also removed that had previously let fragmented opposition parties unite around single challengers to Fidesz.