The European Union says it has begun legal action against the UK over "breaches" related to the Northern Ireland protocol.
Brussels is upset over London's recent move to postpone British exporters to Northern Ireland having to provide export certificates.
Major exporters to Northern Ireland from the British mainland were initially given a three-month grace period to avoid the extra bureaucracy of export certificates, which became necessary after Brexit kicked in earlier this year.
But some supermarkets in Northern Ireland were hit by shortages, due to supply issues brought about by Brexit and the coronavirus pandemic.
The UK unilaterally decided to extend this grace period - which covers areas such as animal products and parcel deliveries - until October.
What is the Northern Ireland protocol?
The UK and EU agreed, before Brexit happened and as part of the post-Brexit deal that there should be no hard border in Ireland.
The border was effectively removed as part of the Good Friday Agreement which was a peace settlement following years of fighting during the Troubles.
After the UK left the EU, the border between Northern Ireland and EU member state Ireland became an external EU border.
To ensure no hard border was put up following Brexit, Northern Ireland continued following some EU rules on trade - but this meant more red tape put in place for certain goods to enter Northern Ireland from the British mainland.
Within the Northern Ireland protocol is the option for either side to invoke Article 16, an emergency move to suspend the free movement of goods between the Ireland-Northern Ireland border.
How has this worked out?
Not well. The system was put to the test almost immediately after the end of the Brexit transition period at the start of this year.
Later in January the EU published plans to invoke Article 16, amid a row with the UK over access to COVID-19 vaccines.
Manufacturer AstraZeneca angered the bloc after it revised down the number of jabs it was going to be able to deliver, despite continuing to deliver on its contractual obligations for vaccine deliveries to the UK.
The EU, therefore, put controls on exports of the vaccine to prioritise member states, and then signalled it was going to override the Northern Ireland protocol to potentially stop the free flow of jabs from Ireland to Northern Ireland.
It quickly backtracked after an outcry, in which the First Minister of Northern Ireland, Arlene Foster, accused the EU of committing “a hostile and aggressive act”.
Commission president Ursula von der Leyen later admitted “mistakes were made in the process leading up to the decision”, adding she “deeply regretted it”.
What does the EU say?
“The UK authorities have authorised individuals to disregard Union law, even though it is directly applicable to them," said the EU's Maroš Šefčovič.
“The Protocol on Ireland and Northern Ireland is the only way to protect the Good Friday (Belfast) Agreement and to preserve peace and stability while avoiding a hard border on the island of Ireland and maintaining the integrity of the EU single market.”
"The EU and the UK agreed 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,” he added.
The EU has now sent a so-called “letter of formal notice” to London complaining it was breaching the deal.
The UK has one month to reply before the EU can start a second phase. The issue could eventually go to arbitration, and ultimately, the UK could be hit with financial sanctions.
What is the view in the UK?
On the UK side, there are some who are now calling for the arrangements over Northern Ireland to be scrapped.
While government minister Michael Gove said the arrangements were “not working at the moment”, the UK government currently rejects the idea of scrapping them.
But it has been accused of exploiting the issue for its own political gain - and of deflecting responsibility for complications that are largely of its own making.
The outcry over the EU’s abandoned plan to invoke emergency measures under Article 16 shows how sensitive the issue is.
The plan would not have led to a "hard" land border, nor would it have stopped vaccine shipments, but it would have meant that export authorisations were needed.
The move drew criticism from all sides.
Michael Gove told a parliamentary committee last week “it was a moment when trust was eroded, when damage was done, and where movement is required in order to ensure that we have an appropriate reset.”