(Bloomberg) -- It may have fallen out of political favor,but a world without migration looks bleak for advancedeconomies.

Analysis of United Nations data by Fitch Ratings shows haltingimmigration would drastically reduce thepotential working population of Group-of-Seven nations, leavingaging societies more dependent on a smaller labor force andresulting in greater financial stress on pension systems andpotentially slower growth.

Related: Shrinking worker pool pressures retirementprograms

Under the UN’s base case scenario – current immigration levelsbeing maintained until 2050, followed by a gradual reductionof half by 2100 – Canada would see its potentialworkforce boosted by 11 percent. But stopping inflows completelywould see it drop by 43 percent in the same period, Fitch says.

Without new immigrants, the U.S. would be faced with a 16percent reduction in its working age population by the end of thecentury, and the U.K. – where concern about levels of foreignworkers featured prominently in last year’s Brexit vote – wouldexperience a 20 percent drop.

While few politicians are as extreme as to suggest atotal ban, U.K. Prime Minister Theresa May aimsto cut annual inward migration to around a third of currentlevels. Meanwhile, President Donald Trump has promisedto overhaul the U.S. immigration system, taking stepsto crack down on certain types of work visa and seekingto deport undocumented immigrants. In Canada, restrictionsimposed by former Prime Minister Stephen Harper toforce employers to hire more Canadians have already causeddifficulties in some industries.

Skewed birth rates across the globe mean between 2015-2020,India alone will account for almost 30 percent of the globalincrease in people of working age.

Restricting the movement of people between those nations withtoo few jobs to keep pace with their birth rate and those facing adecline in the available labor force, will lead to severeimbalances warns James McCormack, global head of sovereignratings at Fitch.

“Politically expedient protective measures undertaken ostensiblyfor the retention of jobs and the promotion of better economicopportunities for residents are more likely to result in weakergrowth and lower national income levels over time. A number ofadvanced economies, including the U.S. and U.K., will be reliant oncontinued immigration in the years ahead to avoid declines inworking-age populations and slower growth.”

Copyright 2018 Bloomberg. All rightsreserved. This material may not be published, broadcast, rewritten,or redistributed.