Senior EU figures have attacked Theresa May’s post-Brexit immigration plan with the president of the European commission, Jean-Claude Juncker, signalling that he expects a row with the British prime minister at an upcoming “moment of truth” summit.

As May sketched out her plans to end freedom of movement and adopt a skills-based migration policy during a Tuesday morning tour of radio and TV studios, there were demands for a tit-for-tat response during a debate in the European parliament.

The plan to curb low-skilled immigration was seized upon by Manfred Weber, the German leader of the centre-right EPP group and a leading candidate to be the next European commission president. He said the UK’s plan to end free movement illustrated the need to stay united against attempts to pick off the benefits of EU membership.

It was not about “punishment”, Weber said, but leaving the EU would have to mean “less growth, less certainty”, for the UK and the Brexiters would now have to face the consequences.

“Brexit means leaving the EU and this means losing the advantages of this union and that is the simple principle,” he added.

Guy Verhofstadt, the European parliament’s Brexit coordinator, said: “We will never accept discrimination based on skills and on nationality.”

In a sign of growing concern over the likelihood of a no-deal Brexit, Weber, whose party is the largest group in the parliament, said that a deal already agreed in talks over citizens’ rights should be ringfenced.

Juncker also expressed concerns over the state of negotiations, telling MEPs: “We will have an interesting meeting [at the October summit] as far as Brexit is concerned. We want a deal. Those who think a no deal would be the better solution are not aware of the difficulties that such a scenario would imply.

“And when it comes to the border between the Republic of Ireland and Northern Ireland, we are sticking to the point of view that we have expressed so many times. Ireland first.”

Later in the debate, stepping in for Juncker, the vice president of the commission, Frans Timmermans, a former Dutch foreign minister, said the EU dearly wanted to limit the damage to relations. But he added: “Let me very clear that carving out parts of the four liberties will not bring a solution…‘If someone says, ‘I want to leave the house but I will take the key and take whatever I want from the house’, that doesn’t work.”

Recent comments from the current and former foreign secretaries, Jeremy Hunt and Boris Johnson, were also seized upon in the debate.

Johnson’s recent calls for a bridge to be built between the island of Ireland and Great Britain were described as “insane” by Verhofstadt, who said the former cabinet minister was more comfortable in “burning bridges”.

On Hunt, who compared the EU with the Soviet Union in his party conference speech on Monday, Verhofstadt added: “But in his case that is not so abnormal – he has once even confused Japan with China so that is not a first time that this is happening.

“That said, the previous time he was insulting his wife but here it was something far more different. He’s insulting not us but millions of ordinary citizens who have lived under Soviet rule for so long time and that’s in fact a point on which he has to apologise, I think, because we cannot take it as such a thing he said.”

Weber also called on Hunt to apologise.

Timmermans said: “Of course at this stage emotions run high on all sides. I have some understanding for that and you have to forget some of the things that are being said with all these emotions…

“But I represent a college of commissioners where more than one third of its members have personal experience of dictatorship, oppression, and dictatorial rule inspired by the Soviet union. They have all fought to liberate themselves from this.”

Timmermans went on: “One of my colleagues, Vytenis Andriukaitis, was born in Siberia. Why? Because his parents, Lithuanians, were deported to Siberia – to the gulag… All these things he told me over the years. Any comparison to the EU is not just misleading it is insulting to what the union is and what we have become”.

The leader of the socialist group in the parliament, Udo Bullmann, said he now backed a second referendum.

“Let me share the criticism on the irresponsible leadership in the UK,” he said. “I believe the majority of people in the UK now know they’re being led down the wrong path and it was a major error to make this decision to leave the community, a major error that the next generation in the UK will suffer from most severely if this is not remedied. If this government fails, and it looks like that, I think the British have a right to say their views on that directly. That’s the message that we must convey in these negotiations.”

The leader of Labour’s MEPs. Richard Corbett, agreed, saying: “People have a right to say this is not what we were told, this is not what we were promised. The issue of Brexit in Britain is not settled. There is still a struggle going on to make sure we are not led over a cliff edge.”