The constitutional court chairman will look at Lisbon again
Czech senators opposed to the EU's Lisbon Treaty have filed a new complaint against it with the country's constitutional court.
The complaint could create a new delay to treaty ratification, even if Irish voters back the treaty in a referendum on Friday.
Czech President Vaclav Klaus, a Eurosceptic, says he will not sign the treaty until the court decides.
The treaty cannot take effect unless all 27 EU member states back it.
The Czech court has rejected previous complaints about the treaty, which is aimed at streamlining EU institutions to improve decision-making in the enlarged bloc.
But it could take the court as long as six months to deliver its verdict on the new complaint, the BBC's Rob Cameron reports from Prague.
Seventeen Eurosceptic senators signed the latest petition, despite the fact that the Czech parliament has approved the treaty.
Aimed at streamlining EU decision-making
Ratified by all member states except Czech Republic, Ireland and Poland
Only Ireland is holding referendum on it
Took a decade of negotiations
Was intended to take effect in January 2009
The senator who lodged the new complaint, Jiri Oberfalzer, told the BBC it centred on persisting concerns that Lisbon infringed upon Czech sovereignty.
He and his colleagues want the court to decide whether the treaty forms the legal foundations for the creation of a European superstate. If it does, they say, then it clearly violates the Czech constitution.
A further threat to Lisbon would emerge if it is not ratified in time for the UK general election, expected next April or May, which the British Conservatives are favourites to win. They have pledged to put Lisbon to a UK referendum if it is not yet in force.
The treaty's opponents say it undermines national sovereignty and concentrates too much power in Brussels. They also argue that it is simply the EU's defunct constitution repackaged.
The Republic of Ireland will hold a second referendum on Lisbon on 2 October.
Irish voters rejected the treaty last year, but EU governments, anxious to bring the treaty into force, then gave Ireland legally binding "guarantees" that Lisbon would not affect key aspects of Irish sovereignty. These Irish guarantees have not yet been attached to the treaty.
Opinion polls suggest that this time round the Irish "Yes" camp will win, despite widespread criticism of the Dublin government over its handling of the economic crisis.
Ireland is the only EU member state to hold a referendum on the treaty, though there have been calls for referendums in several other countries.
Under Irish law, any major amendment to an EU treaty entails an amendment to the Irish constitution - and that requires a referendum.
Elsewhere in the EU, governments argue that Lisbon amends earlier EU treaties and does not need to be put to a referendum.
The treaty would bring in some major changes. It would expand the policy areas subject to qualified majority voting (QMV), rather than unanimity. It would also establish a new post of president of the European Council - the grouping of EU states' leaders - and a high representative for foreign affairs.