A group of 17 Czech senators filed a constitutional complaint against the Lisbon Treaty today, giving eurosceptic President Vaclav Klaus extra time in which to stall signing it into law.
Mr Klaus has previously declared he will wait for the state's Constitutional Court to rule on any complaints about the treaty before signing the treaty.
The court, which has previously rejected an earlier complaint about the Lisbon Treaty - already approved by the Czech parliament - could take months to come to a decision.
The 17 senators, were mostly members of a eurosceptic wing in the rightist Civic Democrat Party that Mr Klaus founded.
"The main point is the question of conflict (of the treaty) with Czech sovereignty," said Jiri Oberfalzer, one of the senators, after filing the 60-page complaint today.
Mr Klaus' spokesman was not immediately available for comment.
The challenge comes ahead of an Irish vote on the EU treaty and European leaders have grown hopeful EU-wide approval of the 2007 treaty was coming closer after various setbacks.
Many believe a Yes vote n Friday's referendum here would add pressure on signatory holdouts Mr Klaus and Polish President Lech Kaczynski.
But if they hold out until a British election, which must be held by June next year, the treaty could yet be in danger.
Britain's opposition Conservative leader David Cameron, leading comfortably in opinion polls, has written to Mr Klaus confirming his intention to hold a referendum on the Lisbon treaty if he wins power and if the treaty is not yet in force.
Although the current British government has ratified the treaty, it is not clear if British voters would pass the treaty in a referendum.