Brexit: What is the Northern Ireland protocol and why is it needed?

Under an arrangement known as the Northern Ireland protocol, goods will not need to be checked along the Irish border when the new UK-EU relationship begins, on 1 January. So how does it work?

The border is a sensitive issue because of the history of Northern Ireland and the agreements made to bring peace, which included the removal of visible signs of the border.

The fear is if any infrastructure were to be installed, such as cameras or border posts, it could become a target and lead to political instability.

The Northern Ireland protocol, negotiated by UK Prime Minister Boris Johnson last October, is part of the withdrawal agreement (which some called the "divorce deal") that saw the UK leave the EU on 31 January 2020.

Under the protocol, Northern Ireland will continue to enforce the EU's customs rules and follow its rules on product standard (known as the single market on goods).

And this will make checks on goods travelling from Northern Ireland (a non-EU country) into the Republic of Ireland (an EU country) unnecessary.

The protocol is due to come into force on 1 January 2021 - the first day of the new EU-UK relationship.

Until then, the UK remains in a transition period with the EU, meaning its trading relationship stays the same as before.

While Northern Ireland will continue to follow these EU rules, the rest of the UK will stop doing so after 31 December.

And that means, in order to comply with EU requirements, some checks will be needed on certain goods entering Northern Ireland from Great Britain (England, Scotland and Wales) - creating a regulatory and customs border in the Irish Sea.

These checks are needed to ensure:

The extent of checks at Northern Ireland's ports is still to be agreed - but some unionist politicians are strongly opposed to this, fearing it damages the UK union.

Depending on the outcome of UK-EU trade talks, it's possible tariffs (taxes on imports) may be charged on goods "at risk" of moving from Northern Ireland into the Republic of Ireland (which remains in the EU).

But if a tariff was paid and the goods stayed in Northern Ireland, the money would be refunded.

A joint committee - made up of UK and EU government officials - will determine which goods are at risk of onward movement to the EU.

The UK and EU are currently negotiating a trade deal that aims to eliminate tariffs on each other's goods and keep checks to a minimum.

However, the Northern Ireland protocol will still be necessary regardless of whether a trade deal is agreed or not - because EU law requires some product-standard checks, even if tariffs are eliminated.

The UK is legally obliged to ensure the goods entering Northern Ireland from Great Britain are checked, according to Jess Sargeant, at the Institute for Government think tank.

"If it looks like the UK is not applying EU law to the EU's satisfaction, it could impose a fine on the UK or take it to the European Court of Justice", she says.

And if companies are not ready to meet the new paperwork requirements, it could lead to delays at ferry ports and trade disruption.

The Northern Ireland protocol replaces the previous plan, known as the Irish backstop, negotiated by former UK Prime Minister Theresa May.

If it had been needed, the backstop would have kept the whole of the UK in a close trading relationship with the EU - eliminating the need for checks along the Irish border.

However, that solution was rejected by many Conservative MPs who feared the UK could become trapped in the arrangement for years.

This would have prevented the UK from negotiating its own trade deals with other countries.