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 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 October 31st 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 per cent of the responsibility for this.”
Mr Juncker, who is not intending to stand for a second term as European 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 comes as campaigns for the European elections are springing into action — with former Ukip leader Nigel Farage due to visit Nottingham on Saturday as part of the campaign for his Brexit party.
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.–PA