The European Union’s efforts to adopt the Lisbon reform treaty faced another setback on Monday when Slovakia said it would block a Czech bid to seek an opt-out unless it too had guarantees that ethnic German and Hungarian deportees would not press legal claims to confiscated property.

“We have only two options. We will either veto the Czech proposal in the EU Council, or we will insist on that it applies to Slovakia as well,” said Robert Fico, the Slovak prime minister, on his website.