DUBLIN (Reuters) - Ireland is willing to examine ways in which a “backstop” to keep the Irish border open after Brexit could be reviewed so long as it does not permit Britain to unilaterally walk away from it, Prime Minister Leo Varadkar said on Monday.

Taoiseach Leo Varadkar speaks to media at the central count centre in Dublin castle at the Irish presidential election and the blasphemy referendum in Dublin, Ireland, October 27, 2018. REUTERS/Clodagh Kilcoyne

The sides in the negotiations on the terms of Britain’s exit from the European Union have signaled progress on agreeing customs arrangements for an emergency Irish border fix but differences persist on the lifespan of the so-called “backstop”.

British Prime Minister Theresa May raised the possibility of a review mechanism for the backstop in a phone call on Monday with Varadkar that she had sought to update him on the current state of the talks, the Irish government said in a statement.

“The Taoiseach (Prime Minister) indicated an openness to consider proposals for a review, provided that it was clear that the outcome of any such review could not involve a unilateral decision to end the backstop,” the statement said.

“He recalled the prior commitments made that the backstop must apply ‘unless and until’ alternative arrangements are agreed.”

May told Varadkar that there would need to be a mechanism through which the backstop could be brought to an end, a spokesman from her office said in a statement.

The phone call followed a report by Britain’s Daily Telegraph newspaper that May’s Brexit Minister Dominic Raab had privately demanded the right to pull Britain out of the backstop after three months.

Varadkar separately told reporters that an expiry date of that nature would not be worth the paper it is written on.

With just five months until Britain is due to leave the EU, May has yet to nail down a divorce deal, with the insurance arrangement to keep open the border between British-ruled Northern Ireland and EU member state Ireland still the outstanding issue.

An open frontier is seen as crucial to preserving the 1998 Good Friday peace accord that ended decades of Irish sectarian bloodshed. But that goal has been complicated by May’s intention to take Britain out of the EU customs union and single market.

Cautious optimism that a deal may be in the offing has also been dampened by uncertainty over whether it would pass the British parliament, deeply split between eurosceptic and pro-EU lawmakers, even within May’s Conservative Party.

May later reiterated to Austrian Chancellor Sebastian Kurz that she believed a withdrawal accord was 95 percent complete, and was “confident” of a deal on the Northern Irish backstop.

But, highlighting the political complexities of Brexit, a large Survation poll for Channel 4 found Britons would vote to stay in the European Union if there were another ballot, backing “Remain” by 54 percent to 46 percent. In the 2016 referendum, Britons voted 52-48 percent in favor of Brexit. [L8N1XG5G8]

“NOT THERE YET”

Irish Foreign Minister Simon Coveney had earlier said that neither Ireland nor the EU would ever sign up to an agreement that could be ended unilaterally. Coveney was later quoted as saying a “middle-ground position” on the backstop arrangement could be found, but added it must be legally watertight.

The latest proposal on the backstop, according to sources in Brussels, would keep all of the United Kingdom in a customs arrangement with the EU, as May has sought. That would include Northern Ireland, as the bloc has insisted.

But London and the EU have still not agreed how long such an insurance policy would last. Britain wants to limit it while the EU says any clear cut-off date attached to the backstop would defeat its purpose.

A senior EU official told Reuters the bloc’s chief Brexit negotiator, Michel Barnier, has yet to inform the 27 states that “decisive progress” has been made in the talks.

Such a recommendation from Barnier is needed for European Council President Donald Tusk to call a special summit of EU leaders to endorse any Brexit deal.

After a meeting in Dublin on Friday with Britain’s Cabinet Office Minister David Lidington, May’s de facto deputy, both Coveney and Lidington said the two sides were “very close” to resolving differences on the border issue.

Lidington also said London would stand by the written commitments it had already made on the backstop, which include the agreement that it would apply unless and until a better solution is found.

The EU has suggested that the tweaked “two-tier” backstop covering all of the UK could give mainland Britain some scope to set its own trade rules - a central demand of Brexiteers - while keeping Northern Ireland aligned with the EU.