BRUSSELS (Reuters) - A “backstop” mechanism for Northern Ireland’s land border with the European Union is a stumbling block for British lawmakers to endorse Prime Minister Theresa May’s Brexit deal with the European Union.

FILE PHOTO: A sign for 'No border' is seen on the border between Northern Ireland and Ireland in Jonesborough, Northern Ireland December 10, 2018. REUTERS/Clodagh Kilcoyne/File Photo

This is what it is and why it matters:

BOTH SIDES WANT TO AVOID A “HARD BORDER” - WHAT’S THAT?

In 1998, Britain and Ireland made the Good Friday agreement to end 30 years of violence in Northern Ireland over whether it should stay British or join the Irish Republic to the south. That ended controls along the 500-km (300-mile) land border and set up all-Ireland rules and institutions that make Northern Ireland special within the United Kingdom. Brexit means possible new checks along the border and a bombing this month by a new anti-British group has raised fears of a return to violence.

SO THE SOLUTION IS...?

Prime Minister May says a special customs and trade arrangement with the EU, to be negotiated during a status-quo transition of 20 to 44 months, will ensure seamless borders between all the United Kingdom and EU -- so there will be no need for intrusive checks on goods at the Irish border.

THAT SOUNDS FAIR - SO WHAT’S THE CATCH?

The Irish government, backed by the rest of the EU, wants an insurance policy in case those future trade talks fail.

AND THAT INSURANCE POLICY IS WHAT?

The Withdrawal Agreement May struck in November with the EU says the UK will remain in a customs union “unless and until” “alternative arrangements” are found to avoid a hard border. The EU had proposed keeping only Northern Ireland in its economic area but May and the pro-British Belfast party that gives her a slim parliamentary majority objected, arguing that it would set Northern Ireland on a course towards union with the Republic.

BUT BRITISH LAWMAKERS OBJECT?

Many dislike the prospect of being bound to EU rules and customs duties that would prevent Britain doing its own trade deals and leave it overseen by EU judges. May has pushed the EU to put a time limit on the backstop, possibly in 2021. Some EU governments have considered such a deadline - perhaps more like 2025 - but Ireland and all other EU leaders have rejected that.

SO WHAT HAPPENS NOW?

No one knows. There is little sign May can reverse the huge defeat the treaty suffered in parliament two weeks ago. The EU says it will not renegotiate the withdrawal agreement or the backstop protocol within it, but could rework the political declaration setting out post-Brexit trade terms that may offer a clearer way of avoiding the backstop -- say by keeping the UK in a customs union. That, however, remains anathema to May.

AND IF THERE IS NO DEAL?

If no one gives ground, Britain will leave the EU on March 29. Ireland and Britain say they have no plan to impose customs controls that would be an immediate target for militants. But the EU has said that no deal means a hard border, of some kind.

SO IRELAND IS IN A BIND?

Very much so. Its EU partners insist they will stand by it and defend the backstop. But they also want to avoid a no deal. If there were no deal, Ireland would not be able for long to let the only EU land border with the UK stand open. If it failed to check goods coming in from Britain, Ireland itself could find the EU raising questions on whether Irish exports to the rest of the Union should remain free of all checks at their ports.