Slovakia postponed ratification of the EU reform treaty after the opposition refused to budge on a threat to derail it, in protest over a government bill.
The centre-right opposition has vowed to walk out if the treaty is put to the vote, even though they back the treaty.
The party said it could not approve the text, with its charter of human rights, because it claimed a government media bill breached those very rights.
The revised EU treaty must be approved by all 27 EU member states.
The Slovak parliament deferred a planned vote on the EU treaty for a day until 1700 (1600 GMT) on Thursday.
Prime Minister Robert Fico said EU states were "shocked" the treaty was being used in internal politics.
And he defied the opposition Democratic and Christian Union party (SDKU) by refusing to give in on the media bill.
Europe's democracy watchdog last week said the media bill would curb press freedom.
The legislation would allow media outlets deemed to be promoting "socially harmful" behaviour to be fined, warned the Organisation for Security and Co-operation in Europe.
The ruling coalition is five votes short of the 90 necessary for the treaty to be ratified in the 150-seat parliament.
The EU treaty has so far been ratified by the parliaments of Slovenia and Malta this month, and Hungary last month.
It replaces a more ambitious constitution that was abandoned after being rejected by French and Dutch voters in 2005, and is designed to streamline the way the enlarged union operates.