Ministers are planning new legislation that could override a key part of last year’s EU withdrawal agreement.
The move could change the nature of new Northern Ireland customs arrangements intended to prevent a return to checks at the border with the Irish Republic.
Downing Street said it was a standby plan in case EU-UK trade talks fail.
Meanwhile, Boris Johnson said that, if a trade deal is not reached by 15 October, both sides should “move on”, which would still be a “good outcome”.
Although the UK formally left the EU in January, it has continued to follow rules set in Brussels during a transition period – which ends on 31 December – while discussions over a long-term trade agreement continue.
Another round of talks – the eighth – begins on Tuesday, aimed at securing a deal to allow companies to trade without taxes or customs checks.
But Mr Johnson is expected to tell EU leaders it must be agreed in time for the European Council meeting on 15 October if it is to be in force by 1 January.
“If we can’t agree by then, then I do not see that there will be a free trade agreement between us, and we should both accept that and move on,” he will say.