Britain’s interior minister Amber Rudd said on Sunday she was looking at a work permits system to control migration from the European Union, responding to Brexit voters’ demand for tighter border controls.

Although formal negotiations on leaving the EU have yet to begin, Britain is searching for a way to satisfy voters who backed leaving the EU because they wanted lower immigration and an end to open borders with the bloc, whilst meeting the needs of an economy in which some sectors depend on foreign labor.

“Work permits certainly has value,” Rudd told the BBC, saying her department was examining immigration control systems and that no decisions had yet been made.

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Britain currently has a visa system for non-EU nationals, but under EU rules citizens from within the 28-country bloc are free to live and work in Britain.

“What we’re going to look at is how we can get the best for the economy, driving the numbers down but protecting the people who really add value to the economy,” Rudd said.

Earlier this month Prime Minister Theresa May rejected a “points-based” system to screen immigrants—something Brexit campaigners promised to implement—stirring fears among some voters that her government was not taking a hard enough line on key issues like immigration.

But May has said the June 23 vote to leave the EU showed Britons wanted to control the movement of people from the bloc.

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Rudd, a close ally of May, backed the government’s long-standing target of bringing net annual migration into Britain, currently at 327,000, down below 100,000.

Migration controls are likely to form one of the most contentious negotiating points in talks with the EU on leaving the bloc, as Britain looks to tighten border controls without losing access the EU single market.

Britain’s EU partners are so far adamant that it cannot enjoy full trade benefits unless it continues to provide free movement for EU nationals.