What is the Irish backstop in Theresa May's Brexit deal and why is it so divisive?

Theresa May's Brexit deal was overwhelmingly defeated in the Commons with much of the opposition centering on the contentious Irish backstop.

MPs fear the UK could become trapped indefinitely in a Northern Ireland backstop but the EU has refused to move on the matter, insisting it "must remain" a part of the deal.

The backstop is the rule that either the Province or the whole of the UK must obey EU rules after the Brexit transition period ends, until Brussels agrees that a hard border with Ireland is not a prospect.

The DUP, which props up Mrs May's Government, has displayed fierce opposition to the backstop, while Boris Johnson has insisted the Withdrawal Agreement does not need to include it.

Below, the Standard answers some key questions on the backstop within Mrs May’s deal.

What was planned for the Irish border?

A backstop solution aims to prevent the establishment of a hard border on the island after Brexit and avoid the need for border checks with Ireland.

It would mean some regulations relating to Northern Ireland remain aligned with the rest of the EU.

It is an insurance policy if no answer can be found through a permanent trade deal after a transitional period from next March but unionists fear it could create regulatory disparity between Northern Ireland and the rest of the UK.

The backstop has been a focus of discontent with Theresa May's draft Withdrawal Agreement with Europe.

The EU says it has no plans to change its negotiating stance and the UK Government has also said it would not accept a hard border in place after Brexit.

What exactly does the backstop propose?

Once the Brexit transition period ends in December 2020, the EU and the UK have agreed the backstop would apply until such a time as a subsequent agreement is in place.

This would create a single EU-UK customs territory avoiding the need for tariffs, quotas or checks on rules of origin between the EU and the UK.

Northern Irish businesses would not face restrictions when placing products on the EU's Single Market.

What has been the Irish Government's reaction?

Irish Taoiseach Leo Varadkar has said it is not possible to renegotiate the backstop without reopening all aspects of the draft withdrawal treaty.

Dublin is keen to ensure a free-flowing border to protect the peace process and north-south trade links.

Varadkar pledged to defend the backstop, but said it was important to remember it was an insurance mechanism and a way of avoiding the return of a hard customs border on the island.

What has been the DUP's reaction?

Party leader Arlene Foster said the Prime Minister must get rid of the backstop.

The DUP is concerned remaining aligned to Europe would create a regulatory divergence between Northern Ireland and the rest of the UK, requiring extra checks on east-west trade and threatening the integrity of the UK as a whole.

MP Sammy Wilson told the BBC that Theresa May's Commons defeat reaffirmed his party's concerns over the backstop and that the Prime Minister must renogotiate with Brussels.

Other opposition to the backstop

Staunch Brexiteer Boris Johnson has previously said the EU would use the backstop to "blackmail" the UK.

He told the BBC’s Andrew Marr Show last year: “The real problem with the backstop arrangement, it gives the power to Brussels and all the other EU states effectively to blackmail us and to get what they want out of the future trade negotiations. It is a diabolical negotiating position to be in.”

Mr Johnson insisted that there could be a withdrawal agreement without the Irish backstop.

Labour leader Jeremy Corbyn said his Brexit deal would not include a backstop and that the DUP clearly disliked it "for very good and sensible reasons".

What have Remain voices in Northern Ireland said about the backstop?

Four pro-Remain parties in Northern Ireland including Sinn Fein and the nationalist SDLP declared it should be "banked" rather than discarded to protect the 1998 Good Friday Agreement.

They said a no-deal Brexit would be "catastrophic".

What happens now?

Mrs May has been told to ditch her red lines on Brexit in order to negotiate a "middle way" on talks.

Europe has so far shown no willingness to reopen months of delicate negotiations and insisted it is up to Britain to come forward with ideas on how to move forward.

The Prime Minister said the risk of an accidental no agreement increased the longer it took to pass a deal, and she now has just three working days to present her 'Plan B' to Parliament.

EU President Jean-Claude Juncker warned that time is running out for the UK to strike a deal, while European Council President Donald Tusk hinted the UK should stay in the EU after the deal was rejected.

A motion of no confidence was tabled by Jeremy Corbyn following Tuesday's vote, with voting to take place on Wednesday evening. If Mrs May loses that vote, there could be a general election.