EU starts legal action against UK over Northern Ireland

"The EU and the UK agreed [on] the protocol together. We are also bound to implement it together. Unilateral decisions and international law violations by the UK defeat its very purpose and undermine trust between us," Šefčovič warned.

The UK has one month to reply to the legal arguments.

Earlier this month, London unilaterally said it would extend the so-called "grace period" for when rules kick in as part of the deal on Northern Ireland.

This includes steps such as a longer exemption from paperwork attached to shipments of food from mainland Great Britain to Northern Ireland's supermarkets and delaying the introduction of sea border checks on food, parcels and pets.

The arrangements on Northern Ireland was one of the most difficult hurdles in the Brexit negotiations.

The final EU-UK deal was designed to prevent a hard border on the island of Ireland by applying checks on goods entering Northern Ireland from Great Britain, effectively therefore creating some checks on the Irish Sea.

An EU official said that it was "crystal clear for all sides" that based on the type of Brexit the UK government had wanted there would be checks on some goods east-west from Great Britain to Northern Ireland.

The new rules have already been delayed by the UK once - until the end of March.

"The UK is actively telling stakeholders not to apply an international agreement - that is a violation," the EU official said, adding that the UK announced delaying the implementation of the protocol without consulting the EU.

The EU official said Britain's unilateral move was in breach of the withdrawal agreement, but said the bloc is open for dialogue with London on finding solutions.

However, the official also added that some issues cannot be overcome as they are the consequences of the UK's decision to leave the bloc, its single market, and customs union.

Two violations in six months

"This is a question of respect for international agreements that you entered into and recently, and when you see two violations in six months, this undermines trust," the official said, referring to the UK's previous stated plans to suspend the rule on Northern Ireland as part of the internal market bill.

The bill was eventually tweaked to respect the Brexit withdrawal agreement.

The EU's infringement procedure launched on Monday could see the UK end up in the European Court of Justice, and could eventually lead to a fine for London.

British prime minister Boris Johnson was in Northern Ireland last Friday (12 March) where unionist first minister Arlene Foster urged him to ditch what she described as "intolerable" arrangements on Northern Ireland.

Johnson said during the visit that the postponement of the implementation of the Northern Ireland protocol was lawful, and that it needed the consent of both communities.

Deputy first minister Michelle O'Neill of Sinn Féin did not meet with Johnson on the visit. Sinn Féin said it had requested a policy discussion with the PM but was only offered media opportunities.

The full protocol should have come into force in January, when the 11-month transition period - which was aimed to prepare for implementing the divorce agreement including arrangements in Northern Ireland and which was not extended by London - expired.