Klaus, a long standing critic of deeper European integration, issued a thinly veiled comparison between the European constitution and communist authoritarianism.
"I am afraid that steps such as the European Constitution endanger freedom, democracy and prosperity," Klaus said.
"This is matter of deep concern because freedom, democracy and prosperity were endangered in the past by other 'isms.' They are now being eroded by another 'ism,' which is called Europeanism and is embodied in the European Constitution."
The Czech Republic has not yet decided whether to hold a referendum on ratifying the constitution, which must be agreed upon by each of the 25 members of the European Union. Technically, a single vote against it by just one member country could lead to it being scrapped.