(BBC) -- The UK has said the temporary arrangements should last for about two years after Brexit in March 2019.
Terms of the transition period, which the UK calls an implementation phase, have yet to be negotiated between the two sides.
The EU says the UK will have to continue to follow its rules and cannot adopt an "a la carte" approach.
The cut-off date for the transition phase is contained in the EU's guidelines for the next phase of Brexit negotiations.
The guidelines state that the UK should continue to follow EU trade law and stay in the European customs union and single market during the transition phase. Rulings of the European Court of Justice will continue to apply, the EU says.
The UK has already said it plans to leave the customs union and single market and end the supremacy of EU court rulings as part of Brexit.
But the precise terms of the transition phase are the subject of the next set of negotiations.