May had already rejected a proposal by the opposition Labour Party that suggested it could vote with the government on an exit plan that would retain a customs union with Europe.
The U.K. leader said membership of the customs union would be “less desirable” than what has been set out and had already been rejected by U.K. lawmakers.
A customs union is an agreement that allows partaking countries to set common external tariffs, allowing goods to travel freely between those countries.
The arrangement allows manufacturers to move goods and parts around the continent — which currently includes Britain and Northern Ireland — without cost or delay.
Remaining in such an arrangement is seen as problematic to those who support the concept of Britain and Northern Ireland setting up its own trade deals with other nations.
As things stand, the legal default is that the United Kingdom will leave the EU on March 29, 2019 with no agreed deal on trade, laws or travel.