Ministers are planning new legislation that would override a key part of last year’s EU withdrawal agreement.
The move would eliminate a requirement for new Northern Ireland customs arrangements which were intended to prevent the return of checks at the border with the Irish Republic.
Labour’s shadow health secretary Jonathan Ashworth accused the PM of backtracking on his promises.
Downing Street said it was a standby plan in case trade talks fail.
Boris Johnson is expected to say later that if no agreement is reached by 15 October, both sides should “move on”.
The prime minister will tell EU counterparts that completing the UK’s exit from the bloc without a trade deal would still be a “good outcome”.
It comes after UK chief negotiator David Frost said the UK was not “scared” of walking away.
Although the UK formally left the EU in January, it has continued to follow rules set in Brussels during a transition period – which ends in 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 in the EU and UK to trade without taxes or customs checks.