Any kind of Brexit will have "negative consequences" which will be worse for Britain than the EU and be entirely the UK's responsibility, European Commission president Jean-Claude Juncker has said.
Mr Juncker said the "ball was in Britain's court" and urged the House of Commons to support the deal negotiated by British Prime Minister Theresa May.
A second extension to Brexit was granted to the UK following talks in Brussels earlier this month, with the so-called flextension meaning the departure date will be 31 October this year, or sooner if the Withdrawal Agreement is passed.
In an interview with the German Funke Media Groupe, Mr Juncker repeated the words of European Council president Donald Tusk and urged the UK "not to waste time".
He said: "We have to be prepared for a soft as well as a hard Brexit.
"In any case, the exit will have negative consequences - for the British more than for the EU.
"There will be no single market-based solution. As I see it, the British side bears 100% of the responsibility for this."
Mr Juncker, who is not intending to stand for a second term as Commission president in 2019, addressed the idea of a more federal Europe saying that the EU should "not become a melting pot in which all differences disappear".
When questioned about the idea of a United States of Europe, he said: "We should give up (using this term).
"I do not believe that we will ever have a centralised American-style state. I do not wish it to happen either."
The interview came as campaigns for the European elections are springing into action - with former UKIP leader Nigel Farage visiting Nottingham as part of his campaign.
Brexit Party leader told the Nottingham rally: "Our intention is to fight these European elections and our intention is to win these European elections."
He added: "We have not formed this party just to protest, just to stick two fingers up to the establishment on 23 May, just to get our own back and tell them what we think of them.
"No, our ambitions are much, much higher than that. I think it's obvious that our two-party system simply doesn't work any more. I think it's obvious that our two big parties serve nothing but their own interests and agenda rather than the nation more broadly."
At the same rally, Annunziata Rees-Mogg - the sister of prominent Tory Eurosceptic Jacob Rees-Mogg - confirmed she would stand as a candidate in the East Midlands region in the 23 May European elections.
"We have got one chance to save this country and to save it effectively from being tied to the European Union, failing as that is with youth unemployment and the rise of the extremists," she said.
"We need to get out, we need to leave. We need to leave cleanly and we need to leave now."
Britons are due to go to the polls to elect 73 MEPs in May after delays to the Brexit process meant the country was bound to return representatives to Brussels, despite the prospect of them having to leave office only a few months later.
When asked about how he would like to be written about in history books, Mr Juncker said "he has tried really, really hard", adding: "Yes that is enough. Not everyone strives.
"Perhaps the addition would be nice: He got stuff done."