The Northern Ireland Protocol has been a source of tension since it came into force at the start of 2021.
The UK and European Union (EU) are holding talks on a way forward.
But disagreements over the protocol have stopped the Northern Ireland Assembly functioning.
What is the Northern Ireland Protocol?
The Northern Ireland Protocol is a trading arrangement, negotiated during Brexit talks. It allows goods to be transported across the Irish land border without the need for checks.
Before Brexit, it was easy to transport goods across this border because both sides followed the same EU rules. After the UK left, special trading arrangements were needed because Northern Ireland has a land border with the Republic of Ireland, which is part of the EU.
The EU has strict food rules and requires border checks when certain goods - such as milk and eggs - arrive from non-EU countries.
The land border is a sensitive issue because of Northern Ireland's troubled political history. It was feared that cameras or border posts - as part of these checks - could lead to instability.
The UK and the EU agreed that protecting the Northern Ireland peace deal - the Good Friday Agreement - was an absolute priority.
So, both sides signed the Northern Ireland Protocol as part of the Brexit withdrawal agreement.
It is now part of international law.
How does the protocol work?
Instead of checking goods at the Irish border, the protocol agreed that any inspections and document checks would be conducted between Great Britain (England, Scotland and Wales) and Northern Ireland.
These take place at Northern Ireland's ports.
It was also agreed that Northern Ireland would keep following EU rules on product standards.
How does the UK want to change the protocol?
The government wants to create red lanes and green lanes for goods imported from Great Britain into Northern Ireland.
The green lane would be for trusted traders transporting goods to Northern Ireland only. These would be exempt from checks and customs controls.
The red lane would be for products going on to the Republic of Ireland and the rest of the EU. These would undergo full checks and customs controls.
Tax rules would also be changed. Northern Irish businesses currently follow EU rules on state aid and VAT. That means government payments to help firms in Northern Ireland, and tax breaks, must be within limits set by the EU.
The UK government wants to remove these limits.
It also wants an independent body to settle disputes over the Northern Ireland Protocol, rather than the European Court of Justice.
Who opposes the protocol?
Unionist parties support Northern Ireland being part of the UK. They argue that placing an effective border across the Irish Sea undermines Northern Ireland's place within the UK.
Northern Ireland's largest unionist party, the DUP, is refusing to take part in Northern Ireland's power-sharing government unless its concerns are resolved.
Even though the DUP came second in May 2022 elections to Sinn Fein - a nationalist party that accepts the protocol - a new Northern Ireland government cannot be formed without its support.
What is the EU saying?
On 15 June the European Commission took legal action against the UK for not keeping to the protocol, and called on the government to return to negotiations.
The EU said it was not prepared to renegotiate the protocol, but has offered to work on how the rules apply, including:
- reducing customs and checks on goods
- reducing the amount of paperwork
- relaxing rules so chilled meats can still be sent across the Irish Sea.
Are the UK and the EU negotiating?
Technical negotiations between the UK and the EU restarted in October 2022.
In January 2023, the two sides reached agreement on sharing data on trade. This will allow the EU to access the UK's IT systems for detailed information about goods flowing from Great Britain to Northern Ireland.
The UK and the EU said they would "work rapidly" on other issues, raising hopes of a possible deal before the 25th anniversary of the Good Friday Agreement in April.