A British government position paper has said that temporary "backstop" arrangements to prevent a hard border on the island of Ireland should not continue beyond December 2021.
Here are some key questions answered.
1. What is the backstop?
Downing Street is planning to prevent a hard border at the UK's only land frontier with a European Union 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 UK government's option of last resort if current talks fail.
Maintaining customs harmony would make border checks less likely or unnecessary, but keeping common food or other regulatory standards covered by the EU single market for goods and services could pose other challenges.
2. What is the British government's latest position?
The UK has said that a "backstop" customs arrangement would be temporary and only in place until a future deal is introduced.
It expects a permanent future arrangement to be introduced by the end of December 2021 at the latest.
3. What would it mean?
Elimination of tariffs, quotas and customs processes on all UK-EU trade.
Applying the EU's common external tariff at the border.
The UK would be able to negotiate, sign and ratify free trade agreements with other countries and implement those elements that do not affect the functioning of the backstop.
4. When would the "backstop" be introduced?
Following the Brexit implementation period, in "specific and narrow" circumstances, such as delay in the implementation of a permanent customs arrangement, and would be time-limited.
The transitional period will last from Brexit day on 29 March 2019 to 31 December 2020.
5. What does this mean for trade?
Common Irish cross-border processes and procedures for VAT and excise, some administrative cooperation and information exchange between enforcement agencies.
Applying preferential tariff rates for trade with the rest of the world as set out in the EU's existing agreements.
Participating in any new EU free trade agreements that come into force during the period of the backstop.
6. Why is Brexit such a difficult political issue for the British government?
Ministers are split over how to manage customs arrangements with the entire bloc, including the Republic of Ireland, long term.
The British Prime Minister has established two groups of ministers to try to reconcile their differences.
7. What options are on the table?
Brexiteers like UK Foreign Secretary Boris Johnson oppose a "customs partnership" with the EU, whereby the UK would collect tariffs set by the EU customs union on goods entering the country on behalf of the bloc.
The other possibility is maximum facilitation and, rather than scrapping customs checks, would use technology to minimise the need for them.
Brexit Secretary David Davis has talked of trusted trader arrangements, which could allow companies to pay duties in bulk every few months, rather than every time their goods cross a border, and automated number plate recognition.