A Government position paper has said that temporary "backstop" arrangements to prevent a hard border between Northern Ireland and the Republic should not continue beyond December 2021.
Here are some key questions answered.
What is the backstop?
Downing Street is planning to prevent a hard frontier at the UK's only land border with an EU state after Brexit if no preferred trade agreement is reached on withdrawal.
Ensuring frictionless passage of goods and services through the border is one of the most vexed issues facing negotiators and the backstop is the Government's option of last resort if current talks fail.
Maintaining customs harmony would make border checks less likely or unnecessary but...