The U.K. wants to remain in the EU's customs union for a three-year transition, according to a government paper to be published Aug. 15, the Financial Times reported.

The U.K. will seek either a new customs partnership, making a customs border unnecessary, or a new "highly streamlined customs arrangement," the paper reported.

But the European Commission said it would not discuss the proposals until it has concluded difficult negotiations on the terms of the U.K.'s exit from the bloc.

The U.K. government is also optimistic that Brussels will allow it to negotiate new trade deals during the transition period, the Financial Times said, citing the document.