Prime Ministers from the Czech Republic and Slovakia both launched a warning shot to Angela Merkel, Emmanuel Macron and Jean-Claude Juncker against pushing EU integration.
The leaders delivered the attack on Mr Macron's plan to "relaunch the EU" on a panel alongside a shocked Donald Tusk, who heads the European Council.
The European leaders were quizzed on the emergence of a Franco-German "grand reform plan" to push EU integration, which would take more power from states.
During the discussion at the GLOBSEC conference this morning, Slovakian leader Robert Fico said Balkan countries would not stand by as Mr Macron and Mrs Merkel reformed the EU.
This comes as the German and French leaders agreed to reopen European treaties and draw up a "common road map" for Europe.
European leaders hit back at the EU chief – sitting next to them
According to the discussions, Mr Macron wants a joint budget, parliament and finance minister for the eurozone.
Speaking at the conference, Mr Fico shot down this idea: "What Europe needs is a good open discussion. We need more respect for our member states.
"I do not want to reopen treaties. It would be wrong for the EU to focus only on itself, on making new institutions, creating posts, taking more power.
"Do not touch or reopen these fundamental treaties. We do not need these discussions. Let's not talk about things that divide us.
"When Europe talks about the obligatory quota for migrants or fining those who don't take in refugees, this is too far.
"If they dare do this, it will throw a rock among pigeons!
"It would be impossible to agree. I would not dare touch the foundations."
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Tusk looked awkward as the European leader tore into the EU plans
The Czech Republic leader agreed with his Slovakian counterpart
They will a rock among the pigeons if they dare to reform the EU
Robert Fico, Prime Minister of Slovakia
Czech leader Bohuslav Sobotka added: "Cosmetic changes are needed, nothing deep like what France and Germany want.
"We should focus on common-sense plans, those useful to our people. We are against opening up the basic treaties.
"We should focus on borders and economic initiatives. Let's not get dragged into negotiations among reforming Europe.
"I do not want the conversation to disrupt the single market. We are a practical nation."
For his part, Donald Tusk dismissed the idea that Mr Macron's plans were controversial.
He added that new institutions and groups were at the heart of the European Union.
Tusk took part in the European panel alongside the PMs of Slovakia and Czech Republic
German Chancellor Angela Merkel pledged to draw up a "common road map" for Europe
Mr Macron, when elected, pledged to "relaunch Europe" and said that treaty change should not be taboo.
His German counterpart, Chancellor Merkel, agreed that she would help on this.
However, this has prompted outrage among smaller EU states.
Even Merkel's right-hand man Wolfgang Schauble called the reforms "too radical" and "not realistic".