The online retailer is concerned that excise duty will now have to be paid twice on shipments of alcohol which are sent from the British mainland across the Irish Sea to Northern Ireland, ITV reported.

Britain left the European Union's single market and customs union on Dec. 31, introducing paperwork and customs declarations for those businesses that import and export goods with the bloc.

In order to keep the border open between the British province of Northern Ireland and EU member Ireland, a separate agreement was struck that requires a regulatory border in the Irish Sea between Northern Ireland and the rest of the United Kingdom.

The British government said the overwhelming majority of parcels continued to move smoothly, and there was no reason for businesses not to send goods to Northern Ireland.

"These goods will not be taxed twice, and we will issue new guidance clarifying the position to ensure any remaining issues are addressed," a government spokesman said.

(Reporting by Andrew MacAskill and Paul Sandle; editing by Costas Pitas and Sarah Young)