BRUSSELS: The EU launched legal proceedings against Britain on Monday, alleging that London had broken the protocol of its Brexit divorce agreement covering Ireland.

The battle marks a bitter new setback to cross-Channel relations just two months after the EU and Britain secured a hard-won trade deal and 15 months after the UK's tumultuous split from the bloc.

EU officials are angry at an announcement by London of a unilateral six-month delay -- until October 1 -- of custom controls on goods arriving in Northern Ireland from mainland Britain.

The EU said this violates the protocol of the 2019 divorce pact that deals with Ireland, one of the most sensitive and fought over issues of Britain's break from bloc membership after 47 years.

The protocol was designed to preserve peace on the island of Ireland by preventing the return of a border between the UK territory of Northern Ireland and EU member the Republic of Ireland.

Prime Minister Boris Johnson agreed to the set-up in 2019, but only reluctantly, as it draws a de facto border within the United Kingdom and keeps Northern Ireland subject to EU rules on goods.

The letter sent by the EU to the UK began an "infringement procedure" that may end up, after a lengthy process, before the European Court of Justice (ECJ), which may impose fines.—AFP