Jean-Claude Juncker, the European Commission chief, has warned that the UK would be "100 per cent" responsible if its decision to leave the EU caused economic turmoil, as he urged MPs to vote for Theresa May's Brexit deal.
In an interview with a German newspaper, Mr Juncker urged the UK not to waste its six month extension to the Article 50 process and added that he did not have hopes of the UK eventually reversing Brexit.
"We need to be prepared for a soft and a hard Brexit. In any event, the UK’s withdrawal will have a negative impact - more for the British than for the EU," he told Funke.
"There will be no single-market-based solution. As far as I am concerned, the British side bears 100 per cent of the responsibility for this."
He added: "We cannot keep on putting off the withdrawal date indefinitely. The best solution would be for the British to adopt the Withdrawal Agreement during the extra time that has been agreed...[Brexit] must one day come to an end."
Economists have warned that Brexit will damage the British economy regardless of whether a "hard" or "soft" approach is adopted as it will create trade barriers with the EU, which makes up 44 per cent of UK trade.
In the same interview, the European Commission president deflected rumours in the British media of his alleged drinking problem, following several incidents where he appeared unsteady on his feet in public.
"I still have sciatica. This is why I have some mobility problems, which the British press often likes to make a big thing about and attribute to other causes.," he said.
"I often limp because I had a car accident. But I am not complaining."
Mr Juncker, in what may prove to be his last newspaper interview as EU Commission chief, made the appeal to British MPs as the UK prepares to elect members of the European parliament, a legal condition of the six-month Article 50 extension.
Brussels fears those elections could lead to a surge in support for far-Right populist parties, such as the Italian Northern League, which are sympathetic towards the causes of Brexit.
In the UK, the vote is also likely to elect swathes of pro-Brexit MPs led by Nigel Farage, whose new Brexit Party could secure nearly a third of the vote according to recent polls.
Mr Juncker went on to say that the EU should stop campaigning for a "United States of Europe."
Asked if the EU would ever become a United States of Europe, he responded: "I last used this term before puberty at the age of 14 years. We should stop using it. I do not believe that we will ever have a centralised state that is comparable to the U.S. I don’t want it either.
"The European Union should not become a melting pot in which all differences disappear." One of the biggest criticisms of Mr Juncker's leadership is his response to the refugee crisis, which led to each EU member state being told to accept quotas on migrants to share the burden.
In his interview with Funke, the 64-year-old stood by the policy but admitted that the EU needed to change its overall approach to migration.
"We need a better Europe on the issue of migration," he said. "Refugees, migrants and illegal immigrants are a pan-European issue. We need a solidarity-based response and a system of redistribution."
Refugee quotas were abandoned by EU leaders in 2017 after fierce opposition from Poland and Hungary, with European Council president Donald Tusk admitting they were "divisive and ineffective."
The UK, however, was never subject to the quotas as David Cameron chose to opt out from the scheme.