Mr Juncker said he and the EU’s chief negotiator Michel Barnier were doing everything they could to secure a deal but if they didn’t succeed the responsibility would lie with Britain.
Warning that a no-deal Brexit would be “a disaster” for both the UK and Europe, he told a German newspaper: “Our chief negotiator Michel Barnier and I are doing everything possible to get an agreement.
“But if we don’t succeed in the end, the responsibility would lie exclusively on the British side.”
Mr Juncker’s comments come as Mr Johnson has faced criticism in the UK for the language he uses when it comes to Britain’s departure from the EU - namely referring to attempts to stop a no-deal Brexit as the ‘Surrender Act’.
Asked on a visit to a hospital in Harlow whether he was looking to find a way to get round the instruction to extend the Article 50 deadline, the Prime Minister said: "No, I must say that we will obey the law but we're confident that we can come out on October 31 and the best way to do that is to get a deal.
"And so that's why the surrender act is so damaging. I won't hide it from you, it has had the effect with our European friends of making them think, 'Hmm, maybe Parliament can block this thing, maybe they will be forced to extend'.
"If you're in a negotiation, that obviously makes it more difficult but we are still cautiously optimistic that we can do it."
Mr Johnson has suggested that he is having positive talks with the EU about coming to a fresh deal, but so far no viable alternative has been drawn up for the Irish backstop - the sticking point in his predecessor Theresa May’s deal.