May's office declined to comment.
"It is not our policy to be in the customs union," a Downing Street official, who spoke on condition of anonymity, said. "It is not our policy to be in a customs union."
The extent of any British post-Brexit involvement in the EU's customs union - which binds members into a trade bloc with common external tariffs - has become an issue of contention inside May's government and her Conservative Party.
Britain has said it wants to secure a tariff-free access deal on trade with bloc as part of the overall agreement May is seeking with with the EU.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
Get the latest election news, live updates and election schedule for Lok Sabha Elections 2019 on ndtv.com/elections. Like us on Facebook or follow us on Twitter and Instagram for updates from each of the 543 parliamentary seats for the 2019 Indian general elections.