LONDON — The U.K. is preparing to throw a new Brexit demand into the Irish border mix: democracy.

According to three senior officials from the U.K. and EU27, Theresa May’s team are exploring ways to insert the “consent principle” into the EU's proposed Northern Ireland backstop — the insurance plan that is intended to avoid a hard border in all circumstances.

The move is part of a two-pronged diplomatic assault to heavily modify the backstop — which both sides agreed to in principle last December — so that May can sell it to her own party and to the Democratic Unionist MPs whose votes prop up her government.

May will address EU leaders over dinner at an informal summit Wednesday, ahead of their discussion without her over lunch the following day.

A senior Number 10 official said May will tell EU leaders their position on the Irish backstop needed to “evolve.”

London is now preparing for a major diplomatic push to overhaul the EU's concept of the backstop.

“We will honour our commitment to ensure that there is a legally operative Protocol on Northern Ireland, but that Protocol must protect the Belfast/Good Friday Agreement in all its parts and respects the constitutional and economic integrity of the UK, which the Commission's proposal does not,” the official said.

May previewed that call for flexibility herself in German newspaper Die Welt Wednesday, and added a warning to the EU27 not to push the U.K. too far. “To come to a successful conclusion, just as the U.K. has evolved its position, the EU will need to do the same,” she said, “Neither side can demand the unacceptable of the other, such as an external customs border between different parts of the United Kingdom.”

In his invitation letter to leaders, Council President Donald Tusk reaffirmed as one of his three points for discussion that the EU27 remains squarely behind the Republic of Ireland on the backstop issue.

"We should reconfirm the need for a legally operational backstop on Ireland, so as to be sure that there will be no hard border in the future,” he wrote.

Despite positive noises coming out of both Brussels and London in recent days, the gap between the two sides on this most fundamental question in the talks remains dauntingly large, exposing the growing risk of an unintended no-deal divorce.

London is now preparing for a major diplomatic push to overhaul the EU's concept of the backstop, according to the three senior officials in the U.K. and EU who spoke on condition of anonymity about the latest thinking in London, Brussels and EU27 capitals.

The EU wants the backstop to be a legally binding guarantee written into the Brexit divorce treaty that ensures Northern Ireland remains closely tied to Brussels whatever economic model the rest of the U.K. pursues after Brexit — thereby avoiding the need for border infrastructure.

On customs, the EU wants the U.K. to agree that Northern Ireland will remain inside the EU’s "territory" should the British government ever decide to diverge to such an extent in trade policy that border checks would be necessary.

May has said this is something “no British prime minister could ever accept” because it would, in effect, create a customs border in the Irish Sea between Great Britain and Northern Ireland, so dissecting the economic and constitutional integrity of the U.K.

Michel Barnier does not accept this characterization and has said he wants to “de-dramatize” the language to ease British concerns. But despite expressing a willingness Tuesday evening to "improve" the EU's backstop offer by finding ways for checks to occur away from the border area, the EU chief negotiator has shown no sign of being prepared to change the fundamental principles of his proposal. "We are clarifying which goods arriving in Northern Ireland from the rest of the U.K. would need to be checked, where, when and by whom these checks would be performed,” Barnier told journalists after briefing EU foreign ministers on progress in the talks.

As part of the de-dramatization efforts, Brussels is willing to let the U.K. police the internal customs border should it ever be established, officials said. In London, this offer is treated with derision. “It’s very gracious of them to let us use our own officials to patrol a border within our own country,” one official said. “But we don’t want anyone doing those checks.”

On regulation, the second element of the backstop, there is more flexibility. The EU wants to ensure Northern Ireland remains tied to EU single market rules and standards across a whole series of sectors necessary for an open border, including food, farming and manufactured goods.

On this, the U.K. is prepared to negotiate — but only if there are new democratic safeguards for Belfast, according to two senior U.K. officials, meaning some say for the Northern Ireland executive when it is reconvened after its current suspension.

“There will have to be some democratic consent (a key element of the Good Friday Agreement),” said one senior U.K. official.

A second official said: “Under the EU’s version of the protocol, it would mean Northern Ireland accepting the EU’s rules without any democratic say. There’s a problem of democratic oversight. You need some kind of democratic anchor if parts of the protocol ever come into force.”

A senior EU diplomat said the EU was “lukewarm” about the U.K.’s Northern Irish "consent" proposal.

Ultimately, while the U.K. government is not prepared to negotiate a special customs status for Northern Ireland, it is prepared to countenance different regulations on either side of the border because this could be achieved without affecting the overall constitutional sovereignty of the U.K., two officials said.

London is relaxed about different agriculture and food regulations applying in Northern Ireland to the rest of the U.K., because they already exist. The regulation of manufactured goods is trickier because there is currently no difference between Northern Ireland and the mainland, but such regulations could be tolerated if elected politicians have a say in their application.

“Regulatory checks are very different to customs checks,” said one senior U.K. official. “They are a different kettle of fish entirely.” Brussels, by contrast, says there can be no differentiating between customs and regulations.

A senior EU diplomat said the EU is “lukewarm” about the U.K.’s Northern Irish "consent" proposal, which is yet to be formally presented. The concern in Brussels and other EU capitals is that allowing Belfast’s “consent” for future rule changes would be a hostage to fortune which could make the crisis caused by Belgian region Wallonia refusing to ratify the EU-wide trade deal with Canada look tame by comparison. The EU will not allow a situation where a third country can hold up its own regulatory changes.

Are Northern Ireland’s leaders really “reliable partners?” asked one EU27 diplomat.

Northern Ireland, after all, has now been without a ruling executive for longer than Belgium was without a government in 2010-2011 — and there is no sign of this changing before Britain leaves the EU in March. “London is going to have to make this decision,” the diplomat said.

On this most fundamental of Brexit questions, the gap between the two sides is still vast.

This article was updated on 20 September to clarify the form of democratic consent for regulator changes that is being proposed by the U.K. government.