The European Union has said it is weighing its "next steps" after Britain formally responded to Brussels over the launch of legal proceedings alleging London broke the Brexit protocol covering Northern Ireland.

"We can confirm that the United Kingdom replied on 14 May to the European Commission's letter of formal notice, sent on 15 March," a European Commission spokesperson said.

"The Commission will now assess the contents of the letter before deciding on next steps."

The EU began an "infringement procedure" in March after the UK unilaterally delayed until October the introduction of custom controls on goods arriving in Northern Ireland from Britain.

Brussels says this violates the protocol of the 2019 divorce pact that deals with Ireland.

The two sides insist they are working hard to find a solution after the protocol helped generate a leadership crisis in Northern Ireland.

The protocol is designed to prevent the emergence of a "hard border" on the island of Ireland.

The special arrangement shifts customs and regulatory checks to Northern Irish ports on goods arriving from mainland Britain, effectively keeping the UK region within the EU's customs orbit.

Earlier this week the UK's lead minister on the Northern Ireland Protocol said it was "not sustainable" if it continued to operate as it currently does.

The dispute over the Northern Ireland protocol is one of multiple feuds souring post-Brexit ties between the EU and its former member - with France and Britain also recently squaring off over fishing rights around the Channels Islands.