Klaus is the last head of state in the 27-nation EU to stand in the way of ratification of the new treaty, aimed at streamlining decision-making in the bloc and giving it a greater role on the world scene.
Polish President Lech Kaczyński signed the treaty on Saturday, leaving the Czech Republic as the only remaining EU country yet to approve the pact.
Exemption sought
The long-time critic of the treaty stunned EU partners as well as the Czech cabinet last week by demanding that the Czechs negotiate a partial exemption from the pact, similar to those earlier negotiated by Britain and Poland.
Klaus said he was worried the treaty may open the door to property claims by Germans expelled from Czechoslovakia after World War Two. The government said its legal analysis showed the fears were groundless.
But Klaus's aide Ladislav Jakl said on Sunday the president demanded a binding guarantee and a political declaration by the EU, insisting that an easier way forward would not be sufficient.
"The president will not be satisfied by any declaration, but only guarantees for every citizen," Jakl said in a live television debate.
"For him, this condition is fundamental, necessary, unbreachable."
The treaty would give the EU a long-term president and a beefed-up foreign representative, and take away national vetoes in some areas.
Klaus says it would create a European superstate that gives too much power to Brussels and big member states, and has so far refused to ratify it even though parliament has approved it.
Klaus's stance has further antagonised partners of the central European country, which lost some clout when its government fell midway through the Czech term as EU president in March.
Irish-style 'declaration' not good enough
Jakl said Klaus could not be satisfied with a declaration similar to that which the EU adopted to assuage Irish fears that the treaty would touch upon Irish neutrality, taxes and abortion rules.
"This [Irish way] seems to me as an absolutely impossible way forward," Jakl said.
This stance is a further obstacle, because a declaration is much easier to achieve than a legally binding guarantee. Jakl did not specify what form of guarantees Klaus is demanding.
The government, led by interim Prime Minister Jan Fischer, said it would discuss how to tackle Klaus's demand on Monday.
The EU is in a hurry to complete ratification because it needs to appoint a new executive, the European Commission, under the new rules and aims to have the treaty in place by January.
If Klaus drags ratification beyond a British election expected in May next year, the treaty will face the danger of defeat in a referendum promised by the Conservatives if they win power.
(EurActiv with Reuters.)
