Without naming Hungarian Prime Minister Viktor Orban - whose government faced an unprecedented vote later on Wednesday to launch suspension proceedings from the EU - Juncker blasted EU leaders who sought to undermine democracy and the rule of law and rejected complaints from lawmakers that the Commission has been lenient toward Hungary, Poland and other eastern states.
He said Europe will "find a solution to avoid hard border in Ireland and defend all elements of the Good Friday Agreement".
He spoke of regret at Britain's impending withdrawal from the bloc which will mark his five-year mandate and warned Prime Minister Theresa May that the European Union would not compromise its single market to let London pick and choose which rules to obey.
"We respect the British decision to leave our Union, even though we continue to regret it deeply", he said in his speech to the EU parliament in Strasbourg.
The European Union must flex its muscles as a world power, EU chief executive Jean-Claude Juncker said on Wednesday, as he spoke critically of U.S. President Donald Trump's retreat from global engagement.
But he asked the British government to understand that the United Kingdom "cannot be in the same privileged position as a member state" when it leaves. "If you leave the Union you are of course no longer part of our single market and certainly not only in parts of it".
But he paved the way for chief Brexit negotiator Michel Barnier to secure a deal with the British government, ensuring that the EU Commission will not "stand in the way".
The proposals come as European Union nations bicker over who should take responsibility for people rescued in the Mediterranean Sea trying to seek better lives in Europe, even as the number of crossings has declined sharply this year.
"After 29 March 2019, the United Kingdom will never be an ordinary third country for us".
Juncker sent a positive signal on parts of May's Brexit proposal, known as the "Chequers" plan for the country residence where it was hammered out in July.
This would keep a frictionless border for supply chains and maintain the invisible Northern Irish border.
"The United Kingdom will always be a very close neighbour and partner, in political, economic and security terms".