UKIP's Nigel Farage attacked Mr Juncker for his plans for a "European army" and "more Europe"
The president of the European Commission has urged the EU to stick together after the shock Brexit vote in his annual State of the Union speech.
In his address today, Jean-Claude Juncker said Britain should get on with triggering Article 50, which commences the formal two-year countdown to the UK's departure.
He said there was "respect and regret" for the UK's decision to leave the EU but insisted the bloc was not at risk.
However, he warned the decision was a warning that the EU faced a battle for survival against mounting nationalism in Europe.
He said: "Far too often national interests are brought to the fore.
"We shouldn't misunderstand this - European integration must not bow to the interests of the nation state. Europe cannot become a colourless melting pot."
Mr Juncker called for an EU military with its own European headquarters - something the UK had long resisted. He said that the lack of cooperation on military resources was costing EU countries billions.
And he said there must been an acceleration of deeper financial union, another key issue for Brexiteers, and doubling the EU investment scheme.
He said the next 12 months were vital to unite Europe and that he was "convinced the European way of life is something worth preserving".
Responding to the address, UKIP's Nigel Farage attacked Mr Juncker for his plans for a "European army" and "more Europe" in the next 12 months and said he was glad the UK voted to leave.
He said: "It is clear that there are no lessons that are going to be learnt from Brexit.
"Indeed it was the usual recipe for more Europe and in this particular case of course more military Europe.
"A European army, a common defence and already in this house people are saying: 'Well the Americans won't be here any more we will have to stand alone with our own military structure'.
"I don't know about a project of peace. I would have thought this is probably a very dangerous move."
"Declaration of war"
Mr Farage also ripped into chief EU Brexit negotiator Guy Verhofstadt, saying his appointment was a "declaration of war on any sensible negotiation process".
"If you were to think of this building as a temple, well Mr Verhofstadt is the high priest, a fanatic," he said.
"In fact, there is only one real nationalist in the room and it's you because you want flags, anthems, armies, you are an EU nationalist."
Mr Verhofstadt also spoke in the State of Union debate, saying the EU should treat Brexit negotiations as an occasion to make progress as a bloc, rather than take revenge.
"Brexit is not a liability, I see it more as an opportunity," he said.
"Our duty, our responsibility is to make Brexit a success for Europe, for all citizens of Europe. And it is a possibility to end the dramatic complexity of our institutions.
"Brexit is not a matter of punishment, it's not a matter of revenge."
In the days that follow, the 27 remaining European Union leaders will meet at a summit in the Slovakian capital of Bratislava - without the UK at the table.
On Tuesday, in a letter written to leaders ahead of the summit, EU president Donald Tusk warned "giving new powers to European institutions is not the desired recipe" for tackling challenges in the bloc - perhaps an admission that the long-held dream of an ever-closer union is no longer feasible in this political climate.