In an escalation of tensions, the European Commission was preparing on Monday to take the move over the UK Government's alleged breach of the Northern Ireland Protocol.
The EU has suggested the UK may be in breach of international law for extending the light-touch regulatory periods, that were due end this month, until October.
The grace periods cover areas such as supermarket supplies and parcel deliveries to Northern Ireland from Great Britain and mean checks are not yet fully applied.
Downing Street was yet to receive a letter notifying it of the infringement proceedings on Monday morning, but one was expected to be dispatched later in the day.
Prime Minister Boris Johnson has said the extensions were "very sensible", with the government denying there has been a breach of the protocol.
The Northern Ireland Protocol of the Withdrawal Agreement was designed by the UK and EU to avoid a hardening of the border on the island of Ireland when the post-Brexit transition period ended on December 31.
Northern Ireland remained part of the EU's single market for goods, meaning products arriving from Great Britain face EU import regulations.
The first of the grace periods had been due to expire at the end of this month, but the UK has pledged to extend them until October in a move widely welcomed by businesses in Belfast.
Scotland's Constitution Secretary Mike Russell has said trade problems in the wake of the Brexit deal are "far more than teething troubles".