Brussels chief Michel Barnier has accused Brexiteers of trying to "destroy" the EU.
The bloc's chief Brexit negotiator claimed Nigel Farage had another motive for campaigning for Britain to leave than simply regaining sovereignty for the UK.
Mr Barnier said Mr Farage and other Brexiteers wanted to tear the Union apart from the inside and from the outside.
And he warned remaining member states to "take good care" of the Union.
Mr Barnier recounted talks he held with Mr Farage during the negotiations during a meeting of the European Parliament's Foreign Affairs Council.
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He said: "At the end of our discussions, I asked Mr Farage one question.
"I said, you campaigned for Brexit, you won that campaign, we respect that even if it's a matter of regret to us, we're preparing a Brexit treaty, I showed it to him.
""Could you tell me how you see the post-Brexit position? How do you see the future of relations between the European Union and the UK following Brexit? What's your vision?", I asked him.
"And his reply was extremely clear. You should be familiar with this, and remember it.
"He said: "Mr Barnier, after the Brexit the EU will not exist."
"So these people want to destroy the EU from inside, and others from outside. That's why I think we must take good care of the EU."
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Earlier, Mr Barnier warned a no-deal Brexit was "very likely" after last night's indicative votes in the House of Commons failed to find a majority for any way forward.
He added: "We have prepared for no deal. It's not the option I would have gone for. You don't need a negotiator for no deal, do you? You need a negotiator for a deal. And that's what we've done. But we are prepared. We've done some serious preparation for this over a number of months now.
"Brexit has monopolised the European agenda in these recent months, a lot of energy and time has gone into it. And I believe this energy and time has to be devoted to a positive agenda."
