As Theresa May tries to put together the best deal for Brexit possible over the next two years, she faces a potential large loss of leverage from within the UK as Scotland agitates for a new independence referendum. “For a prime minister who on Wednesday proclaimed Brexit as an exercise in self-determination,” Scotland’s first minister Nicola Sturgeon wrote this week, “to now seek to block Scotland’s own right to self-determination would be democratically indefensible.” May put off a demand from Sturgeon and the legislators in Holyrood to begin preparations for another vote on breaking up with Britain, asking for patience until the Brexit process unfolds completely over the next two years.

Sturgeon’s reply? We’ll start without you:

Watch First Minister @ScotGovFM @NicolaSturgeon on why she is formally asking for powers to hold a second independence referendum pic.twitter.com/rrg9ruxR6Q

— Scottish Government (@scotgov) March 31, 2017

Nicola Sturgeon has told Theresa May there is “no rational reason” to block the Scottish government’s request for a fresh independence referendum in her official letter to the prime minister.

In the message seeking powers to stage that vote, Sturgeon said agreeing a deal on a referendum should be “a relatively straightforward process” given that both governments had already gone through it. …

“It is my firm view that the mandate of the Scottish parliament must be respected and progressed. The question is not if, but how,” Sturgeon wrote.

“I hope that will be by constructive discussion between our governments. However, if that is not yet possible, I will set out to the Scottish parliament the steps I intend to take to ensure that progress is made towards a referendum.”

The address does toss a rhetorical bone to May, perhaps as a result of a “business-like” meeting this week between the two leaders. Sturgeon holds out the potential to put off the referendum until later in 2019, by which time the Brexit deal should either be complete or very nearly so. However, Sturgeon wrote to May that work would still need to begin on the referendum well ahead of that time frame, including a public airing of the terms of the deal with the EU. “We are also in agreement that – unlike the EU referendum – the choice must be an informed one,” Sturgeon declared. “That means that both the terms of Brexit and the implications and opportunities of independence must be clear in advance of the referendum.”

That presents a conundrum for both leaders. Sturgeon is essentially admitting in this concession that May’s right — an informed choice on independence cannot take place until after the negotiations have been completed. Does it make sense to start a referendum process two years in advance of that point if it might not be necessary at all? That’s especially true since Sturgeon at the same time promised to help May get the best deal possible even while pushing for independence. It would make more political sense to stick with the principled arguments for Scottish independence, which nearly carried the day three years ago, adding to it the desire to stay within the EU, rather than concede the potential for the deal to change the terms of the debate over independence.

May’s conundrum is more acute. She needs the leverage of the existing United Kingdom and its markets to get the best bargain from the EU. A pending referendum that puts Scotland as a possible independent entity obviously undermines her position — plus offers even more incentive for the EU to take as punitive a position as possible. If it gets bad enough, the EU might hope to force the UK to rescind their Brexit position and keep them yoked to Brussels. May’s going to get squeezed, and she knows it; she just wants to put it off as long as possible.

Self-determination cuts both ways, as May and the UK have discovered, and it may cut a lot deeper than anyone anticipated during the Brexit vote.