The European Union is facing existential threat, thanks to the “forces of populism”, Commission President Jean-Claude Juncker has said in his State of the Union address. But he said the threat could be overcome by doubling down on the European project.

In a speech which was drastically revised overnight to more directly address the British vote to exit the EU, Juncker admitted: “Many are wondering whether Brexit is the beginning of the disintegration process of the EU.”

But he continued: “Allow me to state, we respect and at the same time regret the UK’s decision, but the EU as such is not at risk.”

In his remarks, Juncker reiterated calls for Article 50 to be invoked as soon as possible, a clear dig at the British government which has repeatedly said that it wanted to launch a consultation before going ahead with triggering Brexit.

And he again warned that there can be “no a la carte access to the single market,” sticking to the European line that access to the single market necessitates accepting rules on free movement.

However, Juncker made it clear that the leaders of the European project are alarmed by the rise in national interests within member states, and are taking measures to jump-start the European project in response.

“In September 2015, I stated that the State of the Union left much to be desired,” Juncker said. “I said there was not enough union. That still applies. The European Union still does not have enough union.”

He insisted: “Europe is not going down the path of nationalisation, it can never become that. But there are splits out there and fragmentation exists where we need further effort from the Union.”

An arch-Euro Federalist, Juncker and many of his colleagues see the only plausible response to the existential crisis before them as a rejuvenation of the European project. “We have to get to work,” Juncker told the parliamentarians before him.

That means creating a “fair” Europe, he said; one with a level playing field on tax and workers’ pay, and with social justice at its heart.

In a new initiative, Juncker announced the launch of a youth wing of the European project – a “European solidarity corps to volunteer their help when it is needed most to respond to emergency situations like the refugee crisis.

“By 2020, I want to see 100,000 Europeans taking part,” he said.

The Union is to reinvigorate its efforts to create new jobs, he said, and the amount of European strategic investment fund will be doubled.

In another dig at Britian, Juncker said that Europe “can never accept Polish workers being harassed, beaten up or even murdered in the streets of Essex.”

However, he was in a more conciliatory mood when it came to the issue of migrants, saying that the acceptance of a migrant quota “must be voluntary, it must come from the heart, it cannot be imposed”. The admission is a big step forward for Eastern European countries such as Hungary – which is holding a referendum on whether to accept a migrant quota last month – and who have been overwhelmed by the effects of the migrant crisis.

He rounded off his comments with an elegy for a United Europe, saying: “I am as young as the European project, which is celebrating 60 years in March.

“I have experienced, live through this project and dedicated my entire life. I have done it from conviction.

“My father believed in this as well, he knew how fragile Europe was because he had to fight in the war in his own country against his will.

“Europe has forgotten its past…but ladies and gentleman, our children deserve better.”

His comments were welcomed by fellow arch-Euro federalist Guy Verhofstadt, who has been appointed by the European Parliament to lead on negotiating Brexit.

Joining in on the attack against nationalism, Verhofstadt called it a “cancer,” and called on the EU to implement measures to address the fact that young people, he says, “see competition, free market and free trade as a source of inequality rather than…a source of new opportunity and progress for the future.”

Asked by UK Independence Party MEP Gerard Batten if he agreed with him that Britain should exit the EU immediately, before any negotiations take place, Verhofstadt replied with an emphatic: “Yes!”

Taking to the floor, Syed Kamall, Leader of the Conservatives in the European Parliament, warned: “In June, the British people voted to leave the EU…but the voices of discontent reach far beyond the channel. Don’t be surprised if others ask to follow.

Warning that Europe “is set to cruise control” with leaders who “will not apply the brakes,” Kamall said that Juncker’s speech was “fundamentally the same mantra we’ve heard year after year.

“More military integration, more requests for money from member states, more debt to keep today’s socialists happy.

“The more Europe you build, the more detached its citizens feel,” he said.

Demanding a slimmed down EU, he raised the prospect of a Prime Minister Marine Le Pen in France or a Prime Minister Geert Wilders in Holland – a future which he said should “scare all of us”.

Le Pen, herself an MEP, was unperturbed by the slur, telling the chamber “we have just witnessed the funeral of the European Union.”

Addressing Juncker directly she asserted: “Mr Juncker, your lack of faith in the referendum is disturbing.”

And she disagreed with Verhofstadt that Europe’s youth want more EU, pointing out that her party is popular among French youth.

But her praise was reserved for Brexit, which she said “has broken a taboo.” Predicting that more populist movements will spring up, she appealed to her colleagues: “Let’s be democrats and let people decide their fate.”

The outgoing UKIP leader Nigel Farage, who was instrumental in securing Brexit, also took to the floor to deliver a speech which blasted the European establishment.

“Having listened to what you’ve said this morning, all I can say is I am glad we voted to leave. It is clear there are no lessons that are going to be learnt from Brexit,” he said.