The UK and the EU are set to thrash out their differences over the implementation of the Northern Ireland Protocol.

Brexit minister Lord Frost and Maros Sefcovic, the European Commission vice president, will meet on Wednesday after the latter suggested the EU is finding it hard to trust the UK following its departure from the bloc

Mr Sefcovic said there have been “numerous and fundamental gaps in the UK’s implementation” of the two sides’ trade deal and that the EU will act “firmly” if the UK does not agree on deadlines for complying with its obligations.

In turn, Environment Secretary George Eustice claimed the Northern Ireland Protocol, in the way the EU wants to implement it, would make it impossible for UK producers to sell British sausages to Northern Ireland.

Sky News has taken a closer look at the issues.

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What is the Northern Ireland Protocol?

It is a crucial part of the Internal Markets Bill, which was drawn up to ensure trade between all four UK nations remains barrier-free after the Brexit transition period ended on 31 December 2020.

The Northern Ireland Protocol was put in place to avoid the introduction of a hard border between Ireland and Northern Ireland in the event of a no-deal Brexit.

It states that Northern Ireland will remain part of the UK’s customs territory – so if the UK signs a free trade deal with another country, Northern Irish goods would be included.

However, Northern Ireland will have to stick to some EU rules to allow goods to move freely into the Republic.

Goods moving from the rest of the UK to Northern Ireland will not be subject to a tariff unless they are “at risk” of being moved into the EU afterwards.

Mr Eustice said in 2020 there would need to be “some checks on some goods” and “some customs processes but not customs checks” at the border with the Republic.

Goods coming from Northern Ireland to Great Britain can have “unfettered” access, the Internal Market Bill says. This means goods sold in Northern Ireland will be accepted everywhere else in the UK, but the reverse may not be true.

What has happened since the Brexit transition period ended?

Products from Great Britain entering Northern Ireland have had to undergo EU import procedures at the ports.

An Irish Sea border has effectively been imposed in an effort to prevent a physical border between Ireland and Northern Ireland.

This has resulted in delays and sometimes sparse supermarket shelves.

What are the UK and EU disagreeing over?

Under the protocol, a ban will come into force if the UK and EU cannot agree on new regulatory standards to cover the sale of some products after a “grace period” allowed under the agreement.

In March, the UK unilaterally extended the grace period for supermarket goods and parcels for another six months, after it was due to finish at the end of that month.

The EU launched legal action against the UK for extending that grace period.

It is understood British ministers are now considering a unilateral extension for chilled meats, including sausages and mince, which is due to end on 30 June.

After the grace period, chilled meats produced in Great Britain will not be allowed to be sold in Northern Ireland as they are not from the EU, which has strict restrictions on food products.

Mr Sefcovic said retaliation by the EU would be so extreme it would ensure the UK “abides by its international law obligations”.

Boris Johnson‘s spokesman said there was “no case whatsoever” for blocking the sale of chilled meats.

Lord Frost claims the EU has been “inflexible” over the protocol, something the EU rejects.

The EU has said the UK could align with its animal health and food safety rules to remove the need for 80% of the current Irish Sea customs checks.

But the UK has rejected this, as it says it will tie Britain’s hands in trade negotiations with other countries.

The UK has also accused the EU of failing to engage with its own proposals, especially with the issues people in Northern Ireland are facing.