President Klaus has compared EU institutions to the old Soviet bloc
The Eurosceptic Czech President, Vaclav Klaus, wants a new two-sentence footnote to be added to the EU's Lisbon Treaty before signing it, Sweden says.
The new condition came up during a phone conversation between Mr Klaus and Swedish PM Fredrik Reinfeldt, current holder of the EU presidency.
Mr Reinfeldt said the requested footnote was linked to the EU's Charter of Fundamental Rights.
Poland's president is to sign Lisbon in a ceremony on Saturday, officials say.
Poland and the Czech Republic are the only EU states yet to ratify the treaty, which is aimed at streamlining EU institutions, to improve decision-making in the enlarged 27-nation bloc.
Gavin Hewitt, BBC News, Brussels
The lesson of today is this: You can never be entirely certain that President Klaus of the Czech Republic will sign the Lisbon Treaty until he actually does...
Vaclav Klaus told the Swedish PM that he would abide by the ruling of the Czech Constitutional Court - which has not come yet. Then he dropped his bombshell. He wanted a footnote added to the treaty in relation to the EU Charter of Fundamental Rights.
It is fair to say that no one knew about this condition.
The treaty has a reference to the Charter of Fundamental Rights, which covers a wide range of EU citizens' rights. The charter will become legally binding once Lisbon enters into force, although the UK has an opt-out from it.
Mr Klaus has refused to sign the treaty until the Czech Constitutional Court rules on a new legal complaint against it, lodged by senators allied to him.
According to Mr Reinfeldt, Mr Klaus also wants the new footnote adopted by the European Council, the grouping of EU heads of state and prime ministers.
"I told him this is the wrong message at the wrong time for the EU. I told him clearly it is his ink on the paper that counts and I don't want this to delay the treaty going through as soon as possible," said Mr Reinfeldt, quoted by Reuters news agency.
The Czech president told him that he would sign Lisbon if he got the extra footnote and if the Czech Constitutional Court rejected the senators' legal challenge, Mr Reinfeldt said.
"We need clarification on exactly what he [President Klaus] is asking for," he added.
Mr Klaus's demand came only a day after the Czech Prime Minister, Jan Fischer, said he was confident ratification would be complete by the end of the year.
Polish signing ceremony
The BBC's Dominic Hughes in Brussels says EU leaders will see the latest objection as another delaying tactic.
Creates new post of EU president (President of European Council)
New post of High Representative for Foreign Affairs
More decisions by majority vote, rather than unanimity
Ratified by all member states except Czech Republic and Poland
Only Ireland held referendum on it - twice ('Yes' vote second time)
Took a decade of negotiations
Was intended to take effect in January 2009
They will be unlikely to go along with such a request, which runs the risk of opening up the whole ratification process once again - something they are desperate to avoid, our correspondent says.
Mr Reinfeldt appeared baffled and a little angry about the latest development, he adds.
In Poland, President Lech Kaczynski's chief of staff Wladyslaw Stasiak said "the president will sign the treaty on Saturday at noon (1000 GMT)". EU leaders will attend the ceremony.
Earlier, there had been confusion about the president's intentions, with another aide saying the signing would be on Sunday.
The treaty cleared a major hurdle on 2 October when voters in the Republic of Ireland backed it overwhelmingly, in a second referendum. The Irish had rejected it first time round, in June 2008.
President Kaczynski, a Eurosceptic, had said he would wait for the Irish voters' final verdict before signing the treaty.
EU leaders are anxious to get the treaty fully ratified this year - well before UK elections next spring, which could see a triumph for Conservative leader David Cameron. Many in his party oppose Lisbon and are demanding a referendum on it.
EU governments see the treaty as fundamental to the 27-nation bloc's future success. Without it, they argue, the EU's decision-making processes will remain slow and cumbersome, because they date back to when the EU consisted of only 15 nations.
Opponents see Lisbon as part of a federalist agenda that threatens national sovereignty.