Border checks at Gatwick airport | Oli Scarff/Getty Images Immigration to UK may fall before Brexit Former Downing Street adviser predicts migrants will be put off by drop in sterling.

LONDON — The plummeting value of the pound and the “psychological impact” of the Brexit vote will mean that immigration from the EU to the U.K. will fall sharply well before Brexit actually happens, a former Downing Street economic adviser has predicted.

Jonathan Portes, now a research fellow at the National Institute of Economic and Social Research, said the falling pound meant that the value of a British wage was now between 15 and 20 percent lower against the Polish złoty.

Combined with a post-Brexit economic slowdown in Britain and a broad recovery in the eurozone labor market, Portes said he anticipated EU migration to Britain to fall away from current levels over the next 18 months.

“People coming from Eastern Europe especially take into account not just wages as expressed in sterling but wages as expressed in home currency,” Portes said during a briefing at King's College in London. “Since we’ve devalued the pound by 15 to 20 percent against the złoty, for example, you might expect some future impact on migration flows as Poles and others realize that the pound is not what it used to be.”

He said that the “psychological impact” of the Brexit vote could also push down numbers as potential migrants felt "less welcome here,” although this factor was difficult to quantify, he added.

Official figures in August showed that there were 2.23 million EU workers in Britain, with around 270,000 EU citizens migrating to the U.K. in the year to March 2016.

Prime Minister Theresa May has made clear that regaining full control over immigration will be her priority in Brexit talks with the EU, which will commence after March 2017.

Any fall in EU migration is likely to be politically useful to May. But Portes said it would exacerbate the potential economic damage caused by uncertainty in the wake of the Brexit vote, by reducing Britain's labor supply.

Ending freedom of movement of people from the EU to the U.K., as May has indicated she would like to do, would also increase regulatory burdens on British business and increase the size of the state, Portes added.

“This is not a value judgment – this is baked in to the choices that we are making by ending free movement,” he said. “Free movement is, from the point of view of the people in the Home Office, great. It means you don’t have to worry about the Europeans — in the future they will be a problem, there will have to be a bureaucracy to deal with them.”

However, May has left herself some “flexibility” on precisely what immigration system for Europeans she introduces post-Brexit, Portes said after the briefing.

“She has not painted herself into a corner by, for example, saying that Europeans will be treated exactly the same as non-Europeans which is what some people on the Leave campaign, including some who are now in government, explicitly committed to,” he said.

“She has left herself quite a lot wriggle room to give quite preferential treatment to Europeans. On the other hand, she has made clear that we [in Britain] have to decide who should be able to come here to work — not the ECJ, not a treaty with the Europeans.”