LONDON/DUBLIN/BRUSSELS (Reuters) - Britain's Brexit negotiations with the European Union have repeatedly snagged - and sometimes collapsed - over Northern Ireland. Why?
A hard border with customs, security and passport control between the United Kingdom's Northern Ireland and the Republic of Ireland is unacceptable as it threatens the delicate balance brought by the 1998 peace process that ended three decades of conflict between Irish Catholic nationalists and pro-British Protestant unionists.
That created a problem: will the only land border between the EU and post-Brexit UK become a back door into the bloc's cherished single market?
THE SOLUTION: BOTH WORLDS
Prime Minister Boris Johnson agreed to regulatory and customs alignment with the EU for Northern Ireland only, but with the addition of a consent mechanism through which the Northern Ireland assembly could vote to exit the arrangements.
So while Northern Ireland would formally remain part of the UK customs territory it would also be aligned to EU rules and remain a de-facto member of the bloc's single market.
In effect then, the customs and regulatory border is in the Irish sea - between Britain and Northern Ireland.
Squaring the circle on paper was the easy bit: it remains unclear what checks will be needed, and how they should be administered, on goods arriving in Northern Ireland from Britain.
Cabinet office minister Michael Gove has set out a Trader Support Service to help businesses in Northern Ireland comply with new bureaucracy of bringing in goods from Britain.
There are also questions over state aid. Any state-aid rules made by the British government would apply to Northern Ireland, as it is part of the United Kingdom.
But if those rules broke EU rules, then Brussels would be able to challenge them in Northern Ireland and by extension across the whole of the United Kingdom. Any dispute would be settled by EU courts.
That is unacceptable to Prime Minister Boris Johnson and his top advisers who say they are not scared by a no-deal exit.
The Financial Times reported that sections of Britain's internal market bill — due to be published this Wednesday — are expected to "eliminate the legal force of parts of the withdrawal agreement" in areas including state aid and Northern Ireland customs, according to three people familiar with the plans.
WHAT DOES THE UK SAY?
Britain said on Monday it remained fully committed to implementing the withdrawal agreement it agreed with the European Union, describing proposed changes as limited clarifications.
London wants there to be no need for export declarations for goods moving from Northern Ireland to the rest of Britain and it wants ministers to have the power to step in if required.
The government wants to make sure there is clarification to ensure that EU state aid law will apply in Northern Ireland but not in the rest of Britain. It wants a secretary of state to set out guidance to make clear that is the case.
WHAT DOES IRELAND SAY?
Ireland said that undermining the Withdrawal Agreement would be a very unwise way to proceed.
"International agreements and international treaties have to be honoured and they trump any domestic legislation than any country may pass," Irish Deputy PM Leo Varadkar said.
"That agreement is in place to make sure we don't see the emergence of a hard border between north and south, something we all want to avoid," he said.
WHAT DOES THE EU SAY?
The EU reacted by saying Britain would be shooting itself in the foot if it rolled back on commitments sealed under its divorce treaty with the bloc as it would undermine London's credibility in any international agreements, including on trade.
Should the legal amendments not undermine Northern Ireland's links to the EU's single market or the bloc's own state aid policies, ensure proper controls for customs, live animals and animal products, as well as rules of origin on the sensitive border the bloc might end up not objecting to them.
However, the devil is in the detail as the matter is fraught with multiple political and technical challenges.
(Reporting by Guy Faulconbridge, Elizabeth Piper, William James and Gabriela Baczynska; Editing by Andrew Heavens)