The EU's chief negotiator has set out what the UK should expect from a transition period - stressing that everything will be very much on the EU's terms for the duration.
Michel Barnier also warned there is "only one year left" to reach an agreement over Brexit, urging the UK to put forward concrete proposals on the three withdrawal issues during the fourth round of talks.
Speaking in Rome a day before Theresa May is due to give a major speech in Florence (which the European Commission has not been invited to), Barnier told members of the Italian parliament time was "of the essence".
Six months had already passed since Theresa May triggered Article 50 and any agreement would take six months to ratify, meaning there was just one year in which to find a solution on topics such as the Irish border, transition and the future trading relationship.
"The question facing us over the coming months is serious, but simple: will the United Kingdom leave in an orderly fashion with an agreement, or not? From our side, I repeat once again that an agreement is the best outcome. It is in our common interest," he said.
"The sooner we make real "sufficient progress" on the conditions of the UK's withdrawal, the sooner we can begin discussing our future partnership," he added - nodding to the EU's position that no trade deal will be discussed until the divorce bill details are settled.
"If we didn't do this, and allowed the uncertainty to continue, and pushed these difficult subjects to the end of the negotiations, then we would run the risk of failure in the absence of trust between us," Barnier said.
A lack of progress had been made because the EU was still waiting for "clear commitments from the UK on these precise issues", he said, adding that he would be listening "attentively and constructively to Theresa May's important speech tomorrow in Florence".
David Davis' team should bring forward "proposals to overcome the barriers" during the fourth round of talks next week, he said.
"These questions, as far as I see it, should not be controversial because this is about protecting our citizens, our businesses, our partners, and the credibility of our promises... Once we have clarity on these points, we should also define the precise conditions for a possible transition period, if the British government requests one."
"This period would begin on 30 March 2019, when the United Kingdom is no longer a member of the European institutions, and therefore no longer takes part in the decision-making process," he added. "An important point: this short transition period will be part of the Article 50 withdrawal agreement. Without a withdrawal agreement, there is no transition. This is a point of law."
Barnier - who at one point quoted Machievelli - said that the period could be extended but this would require "existing Union regulatory, budgetary, supervisory, judiciary and enforcement instruments and structures to apply – as recalled in the mandate I received from the European Council, under the authority of President Donald Tusk".
It would not be possible for the UK to have "the same benefits as the Norwegian model but the limited obligations of the Canadian model" - namely a bespoke model along the lines rumoured to be in May's speech - Barnier said.