Hungary's governing party Fidesz has lost its two-thirds majority in parliament after a by-election defeat.
Sunday's by-election saw Fidesz lose a seat in the city of Veszprem to independent candidate Zoltan Kesz.
The so-called "super-majority" allowed Prime Minister Viktor Orban to pass a new constitution and other laws without support from other parties.
Critics of Mr Orban say the majority removed vital checks and balances and threatened Hungary's democracy.
Mr Orban's right-wing party and its junior coalition partners, the Christian Democrats, have held the "super-majority" since their victory in 2010.
In last year's general elections, it was retained by just one seat in the 199-seat parliament.
But Mr Kesz's victory over Fidesz candidate Lajos Nemedi - by 42.6% to 33.6% - cost the party that seat and will limit its ability to pass legislation.
"We have shown a yellow card to the government," said Mr Kesz.
The by-election was triggered by Fidesz's Tibor Navracsics, who won the seat in April, being made a EU commissioner.
Mr Kesz was an independent candidate but had the support of Hungary's left-wing opposition parties, which were heavily defeated by Fidesz in 2010 and have struggled to recover.
Mr Orban has been dogged by allegations of corruption during his five years in office and has seen his popularity slump since attempting to pass a controversial internet tax, which was later scrapped.
Other unpopular laws, including a ban on shops opening on Sundays and the introduction of increased road tolls, have contributed to a significant dip for Fidesz in opinion polls.