The “ludicrous” attack by Jean Claude Juncker came as Prime Minister Theresa May announced the date she will trigger the process to end Brussels rule.
Meanwhile the commission’s chief negotiator tried to raise the stakes by saying the other 27 EU countries should prepare for customs controls with the UK.
The “spiteful” threats stand in stark contrast to the reasonable stance taken by the UK Government seeking a deal which will benefit the UK and EU.
The UK will have “the choice to eat what’s on the table or not come to the table at all”, Mr Juncker told German newspaper Bild am Sonntag.
Mr Juncker predicted that Britain’s experience of withdrawal will bring the other 27 member states closer together, as they will “realise it’s not worth leaving”.
He said: “They will all see from the UK’s example that leaving the EU is a bad idea.
“On the contrary, the remaining member states will fall in love with each other again and renew their vows with the European Union.”
We all know that no deal is better than a bad deal
But Juncker and Barnier’s words were described as “an empty threat”.
It led to calls to respond by Mrs May to simply walking away from the EU if Mr Juncker tries to play hardball with no deal in the knowledge it will hurt the remainder of the bloc more.
Tory Monmouth MP David TC Davies said: “It really is ludicrous. He is just making an empty threat.
“We all know that no deal is better than a bad deal and that it will harm the rest of the EU more than us.”
He added: “If Juncker tries this trick then we should just leave.”
Senior Tory MP Peter Bone said: “Juncker and Barnier are just part of the European elite who are in a completely different world to ordinary people.
“The real issue for them is whether they can survive without the UK not whether the UK can survive without the EU.”
He went on: “They are kidding themselves if they think the UK is the only country that wants to leave.
“This is a dying organisation which is in danger of collapsing.”
He added: “Juncker is a leader who is completely out of touch with the people he is supposed to represent across Europe.
“If we need to leave without a deal then it will still be better than staying in the EU.”
Richard Tice, co-chairman of Leave Means Leave said: “For the President of the EU Commission to make these disparaging comments before negotiations have started is completely inappropriate as well as being illegal according to the wording of Article 50.
“It seems clear that he is determined to pursue a spiteful campaign against Britain, which will impact negatively on the citizens of the EU he claims to represent.”
He added: “No deal is better than a bad deal and if the EU is adamant about offering us such a bad deal, the PM should walk away at the earliest opportunity.
“Adopting World Trade Organisation rules is a perfectly acceptable way for Britain to proceed, it is what we do with most of the rest of the world anyway.”
Responding to Mr Juncker’s comments, Mrs May’s official spokesman said: “We have said many times we are at the start of negotiations and let’s see how it unfolds.”
Mr Barnier indicated that he was preparing for the possibility that Brexit will mean tighter controls at borders with the UK.
As he held a technical seminar with member states to discuss the impact on customs controls and procedures, the Commission negotiator said: “EU27 have to start preparing now for future controls.”
Mrs May’s spokesman responded: “The PM has made clear that our aim in this negotiation is to secure frictionless borders and that’s what we are seeking to achieve.”