Ministers have agreed any "backstop" arrangement to prevent the return of a hard border between Northern Ireland and the Republic after Brexit should not continue for more than a year after the end of the transition period.
A Government technical note said the UK expected permanent customs arrangements to be in place "by the end of December 21 at the latest".
The move came after Brexit Secretary David Davis was reported to be considering resigning unless there was a clear time limit on the temporary customs arrangements which would be adopted if there is no final Brexit deal.
The note said: "The UK is clear that the temporary customs arrangement, should it be needed, should be time limited, and that it will be only in place until the future customs arrangement can be introduced.
"The UK is clear that the future customs arrangement needs to deliver on the commitments made in relation to Northern Ireland.
"The UK expects the future arrangement to be in place by the end of December 2021 at the latest. There are a range of options for how a time limit could be delivered, which the UK will propose and discuss with the EU."