The UK will come to regret its decision to leave the European Union, Brussels' top official has claimed.
European Commission president Jean-Claude Juncker said things "cannot remain as they are" for the UK in its relationship with the EU after leaving the bloc.
Addressing the European Parliament, Juncker was cheered by Eurosceptic MEPs as he noted the UK's departure was due on March 29, 2019.
Responding to their applause, Juncker said the time would come "when you will regret your decision."
In a message to Theresa May, who had hoped her Mansion House speech had provided further information on her plans, Juncker said the EU still needed "more clarity on how the UK sees its future relationship."
He insisted the EU was united over the issue of the Irish border, one of the main issues in the negotiation. Juncker also said "cherry-picking is not going to be possible" in any 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."
Juncker said the controversial "backstop" proposal for dealing with the border between Northern Ireland and the Republic "translates faithfully" the agreement reached between 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."
The European Parliament's Brexit co-ordinator Guy Verhofstadt also called for more details from the Prime Minister.