The European Union executive said the backstop option was the only means identified by both London and the bloc to avoid a return to extensive border checks on the island of Ireland after Brexit.
On Monday, British Prime Minister Boris Johnson sent a letter to the bloc demanding that the EU drop the backstop from the divorce deal.
“The letter does not provide a legal operational solution to prevent the return of a hard border on the island of Ireland,” European Commission spokeswoman Natasha Bertaud told a news briefing. “Our position on the backstop is well known… (It) is the only means identified so far by both parties to honour this commitment.”