The EU's chief Brexit negotiator has warned a transition deal with Britain will depend on securing a legally-binding agreement on earlier commitments given by the UK in the talks.
Michel Barnier said agreement on Britain's orderly withdrawal from the bloc, which includes future citizens' rights, the Irish border, and the £39 billion divorce bill, still had to be finalised.
Speaking in Brussels after EU foreign ministers formally adopted the guidelines for negotiations on a transition agreement, he said it was all part of a "big package" which had to come together.
"In the course of this second phase we will have to translate into legal terms the commitments that we had in the joint report," he said
"These have to be put into legally binding language. That is a sine qua non condition for progress in this second phase.
"It is all a big package. If we have no agreement on the withdrawal issues there will be no transition."
EU leaders agreed in December that sufficient progress had been made on the withdrawal issues for them to move on to the second including the transition arrangements and a potential free trade agreement.
However Mr Barnier said they now needed to know what the UK was looking for in terms of its future relationship with the EU.
"It is very important that the UK voices its position on what it wants in this future relationship," he said.
He stressed that EU law would have to continue to apply in the UK during the transition, including any changes that were agreed during that period, as Britain would continue to enjoy the full economic benefits of EU membership.
"During transition the UK will continue to take part in the single market, to take part in the customs union," he said.
"It will continue to have all the economic benefits therefore it must also apply all the EU rules. The single market cannot be a la carte."