Czech President Vaclav Klaus must not ratify the European Union's Lisbon Treaty until the top court rules on a complaint filed this week against the document, the court said in a letter to the president.

The Czech Republic may soon become the last country in the European Union which has not ratified the treaty, meant to give the bloc a bigger global influence and make it more operational.

Irish voters cast ballots today in a referendum on the reform, and opinion polls suggested it would pass, unlike last year when the country shocked the rest of Europe by rejecting the document.

Klaus, a right-wing eurosceptic who opposes any further EU integration, has been delaying ratification, although the treaty has been approved by both houses of Czech parliament.

He has made clear he would wait for the complaint by a group of conservative lawmakers.

The complaint was filed this week, and the court on that basis ordered Klaus not to complete ratification until the court rules.