The government has published the legislation it says would be needed to make changes to part of the Brexit agreement that governs Northern Ireland.
Ministers have said it is necessary as the arrangement - known as the Northern Ireland protocol - is not working.
But the EU has said any unilateral action by the UK would breach international law and warned it could retaliate with a trade war.
What is the Northern Ireland Protocol?
Northern Ireland is the only part of the UK with an EU land border. Since Brexit resulted in the UK leaving the EU’s single market, the protocol was designed to govern the movement of goods between Great Britain and Northern Ireland.
These arrangements have led to customs checks being carried out on goods travelling between Britain and Northern Ireland, something Boris Johnson had previously insisted would not happen.
This has also inflamed tensions among Northern Ireland’s unionist community, who are angry at a trade border effectively being erected in the Irish Sea.
Ministers say it has also put at risk the 1998 Good Friday agreement, which brought peace to the province after nearly 30 years of bloodshed.
What’s the problem?
Boris Johnson agreed to this deal in 2019. But since then he has said the system is not working as the checks are disrupting trade within the UK.
The government has said trade between NI and GB is “critical” to the economic success of Northern Ireland.
However, many businesses in Northern Ireland back the protocol because it continues to allow frictionless trade with the Republic of Ireland, which remains in the EU.
This has led to political stalemate, with the DUP refusing to form a power-sharing executive with Sinn Fein until the protocol is radically changed to all-but end customs checks between Northern Ireland and Britain.
What does the new Bill do?
The UK says the protocol as currently operating has four issues which need fixing.
1. ‘Burdensome customs processes’
The Bill would allow the government to introduce separate “green” and “red” lanes for goods travelling between Great Britain and Northern Ireland.
Goods staying in Northern Ireland would travel through the “green” lane and be freed of “unnecessary” paperwork.
Goods destined for the EU via the Republic of Ireland would travel through the “red” lane and would be subject to full checks and controls and full customs procedures. The UK says this would protect the EU single market, as Brussels has demanded.
2. ‘Inflexible regulation’
At the moment, goods in Northern Ireland need to comply with EU rules even if they will never enter the single market.
Under the legislation, businesses would have the choice of placing goods on the market in Northern Ireland according to either UK or EU rules.
The government says this is to ensure that Northern Ireland consumers are not prevented from buying UK standard goods, as can happen now.
3. ‘Tax and spend discrepancies’
Under the protocol, Northern Ireland has to abide by EU state aid rules, meaning changes to VAT rates for businesses made by the UK government cannot be implemented in the province. Minister say this has created a “two tier” tax system in the UK.
The government argues that the protocol means people in Northern Ireland are missing out on VAT cuts on energy-saving materials and Covid recovery loans.
4. ‘Democratic governance’
The UK says there is a “democratic deficit” because the European Court of Justice - a court of the EU - settles any disputes over the protocol.
The Bill would change this so disputes are resolved by independent arbitration instead.
Defending the bill, foreign secretary Liz Truss said: “This is a reasonable, practical solution to the problems facing Northern Ireland.
“It will safeguard the EU single market and ensure there is no hard border on the island of Ireland. We are ready to deliver this through talks with the EU.
“But we can only make progress through negotiations if the EU are willing to change the Protocol itself – at the moment they aren’t. In the meantime the serious situation in Northern Ireland means we cannot afford to allow the situation to drift.”
What does the EU say?
The EU has made clear that if the government’s proposed changes came into effect, it would represent a breach of international law and could prompt retaliatory action, raising fears of a trade war.
Brussels insists the UK must stick to the Brexit deal agreed more than two years ago. Negotiations between Brussels and London on possible changes to how the protocol is implemented have also run into the sand.
Micheal Martin, Ireland’s premier, said a move to rip up the protocol would be a “new low point” for UK-EU relations.
“The natural expectation of democratic countries like ourselves, the UK and all across Europe, is that we honour international agreements that we enter into,” he said. “This agreement was ratified by British parliament, it was approved by the British prime minister.”
Is this illegal?
Critics of the government have warned breaching the Brexit agreement, a treaty signed with the EU, would be a breach of international law.
But the British government has rejected this interpretation.
It argues the “doctrine of necessity” provides “a clear basis in international law” for the move.
The UK says it is allowed to “safeguard an essential interest”, in this case the Good Friday Agreement, which is being put at risk by “the non-performance of another international obligation”.
What happens next?
The legislation will face fierce opposition in the Commons – including from Conservative MPs.
An internal note circulating among Tories opposed to the Bill said: “Breaking international law to rip up the prime minister’s own treaty is damaging to everything the UK and Conservatives stand for.”
The government is likely to face an even tougher battle in the Lords, the scene of multiple skirmishes over Brexit legislation in recent years.
However, hardline Tory Brexiteers, as well as the DUP, are likely to support the bill, ensuring Johnson has the necessary votes in parliament to force it onto the statute book.