The EU says no substantial progress has been made in negotiations going into next week's summit. "The British government have been unable to deliver on that", he told reporters.
"But let's be very clear, there will be no withdrawal agreement, no transition agreement and no managed Brexit if the British government do not follow through on their clear commitments in writing to Ireland and the whole European Union as a whole".
He said: "I think it (no deal) is more likely than it was a few months ago but I still don't think it is likely".
He added: "We are so deeply grateful for the remarkable solidarity and support we have received from the European Union institutions and fellow member states, none more so than from President Juncker".
The meetings took place ahead of a European Council summit next week, while Juncker will also address a joint sitting of the Oireachtas.
Lord Adonis, who supports the Best for Britain anti-Brexit campaign, said: "Every day that the Prime Minister spends arguing with her own Cabinet and manoeuvring to rewrite the British constitution is a wasted day when it comes to our negotiations with our European Union partners".
In a triumphant statement, Mark Daly, a member of a top Irish Parliamentary Committee, said "Brexit and the changing demographics of Northern Ireland" are going to end the province's United Kingdom membership.
"Mary Lou McDonald and Michelle O'Neill emphasised to Theresa May the need for a City Deal for Derry as part of the inclusive growth strategy for the wider north west".
Juncker said he was "strongly against any temptation" to try to isolate Ireland and not to conclude a withdrawal deal. I expect that they will express their strong support for Ireland in the Brexit negotiations.
The Luxembourgian, 63, will receive an honorary doctorate from the National University of Ireland and attend an official dinner hosted by Varadkar in Dublin Castle later on Thursday. The EU's chief Brexit negotiator will also speak at the same conference.
"We'll be stepping up our preparations for all possible situations, and that does include a "no-deal" scenario".
"If Mr Juncker was really focused on helping Ireland, he would be encouraging the European Union to focus on getting a post-Brexit trade deal in place. This is a selfish interest".
Brexit has caused bitter divisions over the future of the Irish border.
Tomorrow he will meet Irish President Michael D Higgins and visit the home of the Gaelic Athletic Association (GAA), Croke Park, and the GAA museum.
"This would only be used in very specific circumstances and be time-limited".
But there will be no withdrawal agreement without a fully operation backstop.