LONDON - British supermarket group Sainsbury's is urging the government to simplify border requirements between Great Britain and Northern Ireland as there is still "a lot of uncertainty", its boss said on Wednesday.

Britain is no longer part of the European Union's single market and customs union, but Northern Ireland has a foot in both camps as part of the United Kingdom's customs territory, while still aligned with the EU's single market for goods.

Last month the EU launched legal action against Britain for unilaterally changing trading arrangements for Northern Ireland that Brussels says are in breach of the Brexit divorce deal agreed with London last year. 

Britain has denied that the move undermines the part of the Brexit deal that governs trade to the British province, saying it extended a grace period for checks on goods moving to Northern Ireland to ease their passage.

"There's a lot of uncertainty still to work through," Sainsbury's CEO Simon Roberts told reporters after the group reported full year results. 

"We continue to urge the government to find solutions that do simplify the border requirements so that we can move goods for customers, optimise availability and keep the cost and complexity as contained as possible," he said.

Since leaving the EU's single market at the end of last year, supermarkets in Northern Ireland have seen shortages of some food products.

Sainsbury's operates 13 supermarkets in Northern Ireland and has been using a local partner to source some of the products it was struggling to import.

Roberts said he was encouraged by the British government's move to extend easements.

"That gives the industry time to work through some of the challenges, particularly on the complexity and the process to move goods Great Britain to Northern Ireland," he said.

(Reporting by James Davey; editing by Barbara Lewis) ((james.davey@thomsonreuters.com))