European Commission President Jean-Claude Juncker has said the UK would come to "regret" the decision to leave the EU.
Addressing the European Parliament, Mr Juncker was cheered by Eurosceptic MEPs as he noted the UK's departure was due on 29 March 2019.
Responding to their applause, Mr Juncker said the time would come "when you will regret your decision".
Mr Juncker told MEPs in Strasbourg there was "increasing urgency" to negotiate the UK's orderly withdrawal from the EU.
He said he did not want EU citizens to "fall victim to Brexit" as he stressed the importance of securing their rights and said "cherry-picking is not going to be possible" in the future trade relationship between the UK and EU.
"I would rather have preferred Britain not to have decided to leave the European Union, but anyone who leaves the European Union has to know, frankly, what this means," he said.
"If you decide to jettison, leave behind, the common agreements and rules, then you have to accept that things cannot remain as they are."
In a message to British Prime Minister Theresa May - who had hoped her Mansion House speech had provided further information on her plans - Mr Juncker said the EU needed "more clarity on how the UK sees its future relationship".
He said that as the clock counts down the UK's withdrawal from the EU next year, it is time to translate speeches into treaties and that this was especially important for Ireland.
Mr Juncker said the controversial "backstop" proposal for full regulatory alignment between Northern Ireland and the EU "should not come as a surprise" to the UK because it "translates faithfully" the agreement reached between Mrs May and the EU in December.
He said the EU was ready to work with the UK on its preferred option of the border issue being resolved in the future trade deal, or by other specific measures.
But he added: "We need to receive concrete proposals from the UK first."
He reiterated that Europe stood firm and united behind Ireland, adding that when t comes to Ireland, "it's not an Irish issue but a European issue".
But he was heckled by one MEP who shouted "it is a British issue".
Mr Juncker said: "It is all for one and one for all, that is what it means to be part of this union."
EU's chief Brexit negotiator Michel Barnier said the UK was "closing doors" through Theresa May's red lines and it was time to face "hard facts" about the future relationship.
He told MEPs: "I listened attentively to Theresa May's Mansion House speech which confirms the door is closing itself by confirming the red lines - leaving the single market, leaving the customs union."
Mr Barnier added that it was a "rather surprising idea" to think the EU could accept convergence in some areas "and at the same time open up the possibility for divergence when there is a comparative advantage to be had" for the UK.
"It's time to face up to the hard facts," he said.
Sinn Féin MEP Matt Carthy said the British government had not presented a "single realistic proposal on the Irish border".
He called for it to honour its word by committing to the agreement it reached in December.
The European Parliament's Brexit co-ordinator Guy Verhofstadt also called for more details from Mrs May.
He said: "There was this Mansion House speech by Mrs May, but it was mainly repeating the red lines that we know already."
"[She said] we cannot accept the rights of Canada and then the obligations of Norway", but "I don't think that we have ever presented that proposal to the UK side", Mr Verhofstadt said.
"I think the UK side have to understand that the opposite is also not possible - you cannot have the rights of Norway and then the obligations of Canada."
Mr Verhofstadt said it was time to move beyond "slogans" and "soundbites".
He has put forward a motion to the European Parliament for an "association agreement" to deal with the future UK-EU relationship.
But it insists on a "binding interpretation role" for the European Court of Justice (CJEU) and rejects UK efforts to "cherry-pick" single market access for particular sectors, such as the financial services industry.
Former UKIP leader Nigel Farage claimed the European Commission was trying to "bully" the UK.
Mr Farage said the UK should leave the EU without a transition deal and claimed a transatlantic trade deal could be struck "in 48 hours".
He said: "Mrs May, we did not vote for a transition period, we voted to leave this organisation, we voted to leave the customs union, we voted to leave the single market.
"Please Mrs May, at this summit next week, do what [Donald] Trump has done - stand strong against the European Commission, against the unelected bullies."