UK Prime Minister Theresa May Agencja Gazeta/Przemek Wierzchowski/via REUTERS Britain's prime minister Theresa May strongly implied at the recent Conservative party conference that she intends to end freedom of movement after Brexit — a policy which allows EU citizens to live and work in the UK without having to apply for a visa.

The details of a post-Brexit immigration policy are not yet clear, but ministers have said that they will cut immigration to the "tens of thousands" (total net migration in the year ending March 2016 was 327,000) and focus on reducing the number of unskilled workers coming to the country.

And while Brexiteers talk about "taking back control" and getting rid of Brussels red tape, an immigration expert believes that ending freedom of movement may have precisely the opposite effect.

Jonathan Portes, a fellow at the National Institute of Economic and Social Research, specialises in immigration and labour market research. The full impact of a post-Brexit immigration policy can only be assessed when it is announced in more detail — but any new system which ends freedom of movement will face the same set of challenges, and Portes outlined four of the most significant ones facing May as she looks to "take back control."

1. The policy will reduce the flow of skilled workers.

The first problem, Portes said, is that a tough immigration policy — and the anti-immigrant message it sends — inevitably puts off highly-skilled workers, even if they are allowed to come to the UK in theory.

"We don't choose who comes here," Portes said. "We set the rules and people then choose whether or not they want to come here. The idea that a new system which in theory lets in skilled workers from the EU will not in practice deter a significant number of skilled workers from the EU is pure fantasy.

It's already becoming more difficult to recruit high-skilled European nationals — that will continue

"We've seen this already. Martin Roth, the director of the V&A, has already decided he's going to pick up and go back to Germany, because he doesn't like what's happening here.

"There is plenty of other anecdotal evidence from not just the university sector but also from other high skilled sectors that it's already becoming more difficult to recruit and retain European nationals who are in high-skilled jobs — that will continue," he added.

2. It is impossible to plan a Labour market centrally.

Another problem is the difficulty of planning a labour market centrally.

While May has already ruled out a points-based system, there are still calls within government for quota-based systems. The Spectator's James Forsyth reports that Whitehall is looking at "a quota system for EU migrants that would let in a certain number each year."

Portes said: "There are a lot of people saying: "We just want control. We want all these skilled Europeans, we just don't want the unskilled ones. Surely we can have a system that just lets us have that?"

You can't decide exactly who's going to be a contributor and who's not. There's a lot of randomness in human behaviour

"There are people in the Conservative party, or UKIP, and perhaps even in the Labour party, who think you can plan the labour market centrally," he said.

"But you can't decide, and pick, exactly who's going to be a contributor and who's not. There's a lot of randomness in human behaviour."

3. Increased regulations and burdens on businesses.

Brexit has been widely hailed as an end to the red tape and bureaucracy associated with Brussels by Vote Leave supporters. But the end of freedom of movement will in fact mean the start of a huge new layer of regulation.

"Whatever new system we have will increase regulatory burdens on business, increase bureaucracy, and increase the size of the state," Portes said.

"This is not a value judgment. This is simply baked into the choices that we are making by ending free movement. From the point of view of the Home Office — the people who actually have to record immigration statistics — free movement is great, because it means they don't have to worry about Europeans.

In future, they are going to be a problem. And there is going to have to be extra bureaucracy to deal with it."

Any new system will increase regulatory burdens on business, increase bureaucracy, and increase the size of the state

4. There will be an increase in the number of illegal immigrants.

This is — again — not something that most of those who voted for Brexit had in mind.

"Whatever new system we have will almost certainly increase illegal working," Portes said.

"We're not going to stop people coming here, we're only going to try and stop them working illegally. Most illegal working the UK comes from people who overstay — people who come here from America or Australia, notionally on a tourist visa, and stay on working without a legal permit to do so.

"In future, that will apply to Europeans."

As for the economic damage the new system might have, Portes said that would be impossible to assess until more policy details had been announced.

"The implication of what I've said is that there will be some economic damage from all of this, but as yet we don't know how much," Portes said.

"We won't be able to quantify it. There are choices to be made about where, on the spectrum, we end up [how strict or flexible the policy is]. We have chosen to go down a road in immigration terms that almost certainly reduces the flexibility."

"How much damage it will actually do is still up for grabs."