Immigration remains a daunting policy concern as the United Kingdom tries to envision a future outside of the European Union. But hopes that Britain and the EU can reach a deal addressing the nation’s migrant population could be in vain, according to a leading academic.

For one, “no conceivable Brexit deal will involve expelling” Eastern Europeans currently residing in the U.K., said Paul Collier, professor of economics and public policy at the University of Oxford.

Many are keeping an eye on how Prime Minister Theresa May handles the issue in upcoming rounds of Brexit negotiations. May has pledged to “re-take control of the U.K.’s borders,” which includes measures like implementing a migrant registration system.

Her hands could be tied, however, when it comes to handling the millions of non-citizens already living and working in the country.

“Ten years ago, Britain made a huge mistake. It was the one country in the EU that said, ‘We won’t impose a seven-year transition period, we’ll just open up our borders,” Collier said.

The move led to a large influx of people into Britain and there is “no way on Earth that’s going to reverse,” he added.

Some analysts have singled out immigration as a factor leading to the Brexit vote. Anti-immigration sentiment appears to be on the rise not only in Britain, but in other European countries including Germany, Austria and the Czech Republic.

About 3.6 million citizens of other EU countries resided in Britain in 2016, making up about 6 percent of the nation’s population, according to statistics from U.K.-based independent fact-checking organization fullfact.org.

In 2015, refugees in the country numbered at about 123,000, according to fullfact.org.

Nonetheless, solving Europe’s refugee crisis at large remains possible, according to Collier. The method lies in creating jobs for the people in refugee havens like Jordan, said to be especially open and hospitable to immigrants — an effort that European countries have largely neglected, he said.

It must be noted, however, that the situation for Syrian refugees in Jordan has more challenges than just jobs.