By Laurence Norman and Valentina Pop 

BRATISLAVA, Slovakia--European Commission President Jean-Claude Juncker called out his critics on Friday, saying the European Union's executive is driving through "modernizing" reforms and that there is no consensus behind any alternative approach.

Criticism of Mr. Juncker has broken into the open since the British vote to leave the bloc with officials and diplomats in Brussels and in some capitals complaining of a lack of leadership and too much effort by the commission to impose its agenda on member states.

Several EU diplomats have said there was discussion in some EU capitals that Mr. Juncker should consider his position, an idea Mr. Juncker has already dismissed.

Mr. Juncker said his commission had discarded red tape and focused the executive's work to drive forward work on deepening EU capital markets, an energy union and removing obstacles to the bloc's single markets.

"I would like those who are watching us to take that into account when they are asking for a huge reform process in the European Union," he told reporters alongside Slovak Prime Minister Robert Fico. "Everyone is saying we need more reform but nobody is saying what kind of reforms we would need in addition to those which are under way."

Mr. Juncker added: "I will not say that nothing has to change but things moving into the right directions will not change."

Mr. Fico has been among those pushing for a post-Brexit rethink of the EU to return authority to the member states to correct what he called Thursday the "failed" policies of the past. Slovakia and several other central and Eastern European member states have been antagonized by EU efforts to force all countries within the border-free Schengen zone to accept asylum seekers that have arrived in Europe.

Mr. Fico will host a summit of EU leaders in Bratislava on Sept. 16 to prepare for the bloc's negotiation on Britain's exit and to discuss ideas for how the EU can respond to multiple crises.

The Slovak leader said Friday the aim of the summit wasn't to weaken the hand of the European Commission and other institutions.

"There had to be a balance, an equilibrium between member states and European institutions. Neither should dominate," he said.

Underlining the uncertainty over how to rejuvenate the bloc, Mr. Fico said the September gathering could be a "breakthrough" summit, even though he said there were no plans for any "radical steps."

Mr. Juncker also sought to play down the conflict with Mr. Fico on migration and his EU reform plans.

"I didn't feel that it's the intention of the Slovak government to enter into war with the European Commission," he said.

Mr. Juncker addressed the British referendum vote and the reasons behind it. He warned the U.K. government once again that there would be no free access to the bloc's single market once Britain leaves unless the U.K. accepted the principle of the freedom of movement of EU workers.

"I have had so many discussions before and after the referendum to know that there was one major issue--that was the freedom of movement of workers," he said. "I will not change that because this is a basic freedom of the European Union."

"If you want to have access, like the other member states, to the internal market, you have to accept all the consequences of the four freedoms which are underlying this internal market thus and mainly the freedom of movement of workers," he said.

Criticism of Mr. Juncker's leadership predated the U.K. referendum on EU membership, but calls for his resignation have multiplied since.

Earlier this week, two ministers for the first time publicly called for Mr. Juncker's resignation. Czech foreign minister Lubomir Zaoralek and his Polish counterpart Witold Waszczykowski claimed Mr. Juncker isn't the right man for the job and should quit. Poland has clashed with the European Commission over what Brussels described as attempts to take political control of the constitutional court and curb media freedom, while the Czech Republic--together with Hungary and Slovakia--has been at odds with the commission over the bloc's migration policy.

Hungarian Prime Minister Viktor Orban said Wednesday after a meeting of EU leaders in Brussels that while he has his "own opinion over the performance of Mr. Juncker," he won't call for his resignation. But he said Mr. Juncker's appointment as commission chief in 2014, overruling objections from the U.K. and Hungary, was one of the reasons why British people voted to leave.

Within the European Commission and in other EU institutions, unease is growing amid some claims that Mr. Juncker's only listens to big countries and is increasingly aloof. "He may not resign now, but at one point things will get out of hand," said one EU official.

Write to Laurence Norman at laurence.norman@wsj.com and Valentina Pop at valentina.pop@wsj.com

(END) Dow Jones Newswires

July 01, 2016 08:57 ET (12:57 GMT)

Copyright (c) 2016 Dow Jones & Company, Inc.