The government is commissioning a "detailed assessment" of the costs and benefits of EU migrants as it plans how to manage immigration after Brexit.
A new set of rules is needed for when EU free movement ends in the UK.
Home Secretary Amber Rudd has asked the Migration Advisory Committee to study current migration trends and asses the impact of a cut in numbers.
She said it would be a "major step in ensuring we create a system that works in the best interests of the country".
The CBI said businesses "urgently" needed to know what this would look like, both in any "transitional" period after March 2019 and beyond.
And the Liberal Democrats said the study had been commissioned a year too late.
Immigration was one of the central topics of last year's EU referendum campaign, and ministers have promised to "take back control" of the UK's borders as they negotiate Brexit.
They have also promised to bring total net migration - the difference between the number of people moving to the UK and the number leaving - from the EU and the rest of the world to below 100,000.
It was 248,000 last year.
An immigration bill, which will reveal the government's chosen method, was included in last month's Queen's Speech.
The Home Office has asked the advisory committee to consider the regional distribution of EU migration, which sectors are most reliant on it, and the role of temporary and seasonal workers.
The committee will also study the "economic and social costs and benefits of EU migration to the UK economy", its impact on competitiveness, and whether there would be benefits to focusing migration on high-skilled jobs.
Ms Rudd said:"We will ensure we continue to attract those who benefit us economically, socially and culturally.
"But, at the same time, our new immigration system will give us control of the volume of people coming here - giving the public confidence we are applying our own rules on who we want to come to the UK and helping us to bring down net migration to sustainable levels."
She said there would be an "implementation period" when the UK leaves the EU to avoid a "cliff edge" scenario as new rules kick in.
Lib Dem home affairs spokesman Sir Ed Davey said:"This will do nothing to reassure the hospitals that are already seeing record numbers of EU nurses leaving, or the companies struggling to recruit the staff they need.
"The NHS, businesses and universities that depend on European citizens need answers now, not in another 14 months' time."
The CBI said commissioning the report was a "sensible first step", adding:"Workers from across Europe strengthen our businesses and help our public services run more smoothly - any new migration system should protect these benefits while restoring public confidence."
Manufacturers' organisation EEF said the migration committee was "best placed" to advise on what EU migration should look like after Brexit.
"Many manufacturers will see today's announcement as a first step, with the government for the first time acknowledging that future migration changes will be implemented in a measured way over a period of years," it said.
Both EEF and the CBI called for an immediate resolution of the question of the status of EU nationals already living in the UK....