Like so many countries in the developed world, the U.K. is on a crusade to close its nagging gender pay gap. Between male and female full-time employees last year, the gulf was 9.4%—down from 17.4% in 1997. For all employees, the gap was 18.1% in 2016, down from 27.5% nearly two decades prior. Even at the current rate of progress, one study predicts it take until 2069 for men and women to reach pay parity.
On her first day in office, Prime Minister Theresa May included the lingering pay discrepancy among the “burning” injustices she pledged to defeat. And later this week, the government will implement a naming-and-shaming law that requires large companies to publish their gender pay gap, their gender bonus gap, and the gender makeup of employees in each quartile of their pay structure every year.
But there’s worry that—like so many facets of British life—the U.K.’s drive for equal pay could be disrupted by the giant whirling wrench that is Brexit. EU directives dictate equal pay, and the U.K. last week officially started its split from the bloc.
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Labour MP Jess Phillips posed the resulting concern to experts rather bluntly during a Women and Equalities Committee session in September: “If we were to leave [the EU] tomorrow—if you were to accidentally trip over something and trigger Article 50 while you are here—what would that mean for equalities in this country?”
Now that the U.K. government has officially launched the Brexit process—and since Tuesday marks Equal Pay Day in the U.S.—it’s worth re-examining the answer to Phillips’ question.
In short, Brexit will not erase Britain’s equal pay protections immediately, but the process could eventually demolish what’s been a reliable legal backstop.
The U.K. actually adopted its own Equal Pay Act in 1970 before joining the EU in 1973. “We were ahead of game,” says Catherine Barnard, a European Union law professor at the University of Cambridge. Yet, as Barnard told the Women and Equalities Committee this fall, the EU has “had a dramatic influence” on equal pay in the U.K. by adding teeth to the initial law Britain had in place.
For instance, until an EU directive in 1975, the U.K.’s Equal Pay Act only applied to equal work, not work of equal value. A female housekeeper could only compare her pay to that of a male housekeeper. The EU directive, meanwhile, held that a female housekeeper can also compare her pay to that of—say—a male gardener since both employees’ work has the same value; it requires the similar skills, responsibilities, and decision making. A ruling in the European Court of Justice led the U.K. to amend its Equal Pay Act in 1983 to incorporate the concept of equal value. The U.K. Equality Act that became law in 2010 provided even more protections.
Barnard says when Brexit is complete some two years from now, it will not immediately affect these U.K. laws because Parliament—the body that passed them—would have to act to roll them back.
But the very possibility of repealing equal pay laws—or other worker rights, for that matter—is the one big change Brexit could usher in. For the past four decades, the U.K.’s membership in the EU has guaranteed that Britain maintained at least the minimum protections set forth by the bloc. If the deal extracting Britain from the EU fails to implement the bloc’s social laws in some way, that floor could drop out from under Brits once the U.K. finalizes its split.
For her part, May has vowed to “ensure that workers’ rights are fully protected and maintained.”
“Indeed, under my leadership, not only will the government protect the rights of workers’ set out in European legislation, we will build on them,” she said in a speech in January.
But if the economy crumbles post-Brexit, it’s possible that political priorities could shift toward sparking growth—and that regulations like equal pay laws could be seen as restrictive red tape.
A version of this story first appeared in Fortune’s World’s Most Powerful Women newsletter. Subscribe here.