A group of Czech senators are taking their fight against the Lisbon treaty to the country's constitutional court today, in one of the last remaining obstacles to its complete ratification.
Of the EU's 27 member states, only the Czech Republic has not yet ratified the treaty, which needs unanimous ratification to come into effect.
If the Czech constitutional court rules the treaty is unconstitutional because it infringes national sovereignty, president Vaclav Klaus' opposition to Lisbon could derail the process.
That would give Tory leader David Cameron the time to instigate a referendum on the subject if he wins the next election, potentially bringing the entire European project to a standstill.
Mr Cameron has promised a referendum only if the treaty has not yet been fully ratified across the EU.
But if it throws the case out, pressure from other European nations will make it much harder to continue to hold out.
Mr Klaus, a natural eurosceptic, said last week his reluctance to sign the treaty because of concerns it could open up Czech citizens from property claims by Germans ousted after the second world war had been addressed.
Sweden, which currently holds the rotating presidency of the EU, has agreed the Czech Republic could agree an opt-out to the Charter of Fundamental Rights contained in the Lisbon treaty.
An immediate ruling appears unlikely, Czech prime minister Jan Fischer said yesterday.