The European Union said on March 15 it is starting legal action against the United Kingdom, arguing it does not respect the conditions of the Brexit withdrawal agreement and is violating international law.

The 27-nation EU is objecting to Britain unilaterally extending a grace period beyond Apr. 1 that applies to trade on the island of Ireland, where the EU and the United Kingdom share a land border and where a special trade system was set up as part of the Brexit divorce deal.

It marks yet another worsening of relations between the two sides since a divorce transition period ended on Jan. 1. Disputes have ranged from fights over vaccines, to the full diplomatic recognition of the EU in Britain and now again the terms of the divorce agreement.

Britain announced last week that it is delaying the imposition of checks on some goods from the EU to give businesses more time to prepare for new post-Brexit rules.

And on March 3 the U.K. decided to unilaterally extend a grace period until October on checks for goods moving between Britain and Northern Ireland. Northern Ireland is part of the United Kingdom but remained part of the EU’s single market for goods after Brexit to avoid a hard border that could revive sectarian violence.

That means that products arriving from Britain face EU import regulations.

In September last year, the U.K. had already upset the 27-nation bloc when it considered legislation that would have given Boris Johnson’s government the power to override part of the Brexit withdrawal agreement relating to Northern Ireland.

So the EU sees the U.K.’s March 3 statement as proof that Britain has now twice sought to violate an international agreement.

The sensitivity of Northern Ireland’s status was underscored this year when the EU threatened to ban shipments of coronavirus vaccines to Northern Ireland as part of moves to shore up the bloc’s supply. That would have drawn a hard border on the island of Ireland - exactly the scenario the Brexit deal was crafted to avoid.

The grace periods cover areas such as supermarket supplies and parcel deliveries to Northern Ireland from Great Britain and mean checks are not yet fully applied.

The first of the grace periods had been due to expire at the end of this month but the UK has pledged to extend them until October in a move widely welcomed by businesses in Belfast.