Brexit in any form will more "negative consequences" for Britain than the EU, European Commission president Jean-Claude Juncker has claimed.
Mr Juncker said the "ball was in Britain's court" when it came to Brexit and said the outcome of any Brexit deal will be the UK's responsibility.
The 64-year-old urged the House of Commons to back Mrs May's Brexit deal - which has already been rejected multiple times by MPs.
His intervention comes after a second extension to Brexit was granted by the EU this month, meaning the so-called "flextension" could lead to Britain's departure by October 31 this year, or sooner if the Withdrawal Agreement is passed by Parliament.
In an interview with Funke Media Groupe in Germany, Mr Juncker repeated the words of European Council president Donald Tusk and urged Britain "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 thought to be standing for a second term as commission president later this year, was quizzed on the idea of a more federal Europe, stating he does not believe the EU should "become a melting pot in which all difference disappear".
He also said "we should give up" using the term United States of Europe,
British MEPs currently serving in the European Parliament
Brexit Party: 14
UKIP: 3 (two of which sit with the Europe of Nations and Freedom group)
Scottish National Party: 2
Sinn Fein: 1
Liberal Democrats: 1
Ulster Unionist Party: 1
Democratic Unionist Party: 1
Plaid Cymru: 1
Social Democratic Party: 1
He added: “I do not believe that we will ever have a centralised American-style state. I do not wish it to happen either.”
Mr Juncker's remarks come as European election campaigns are about to get underway, with Brexit party leader Nigel Farage due to visit Nottingham later on Saturday.
Britons will head to the polls in May to elect 73 MEPs to represent them in Brussels - only for them to leave office a few months later if Brexit deal is agreed upon.
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.”