In London alone, almost 700,000 foreign-born residents are believed to have moved out, leading to a potential 8 per cent shrinking of the size of the capital, it argues.
The study, by the government-funded Economic Statistics Centre of Excellence (ESCoE), draws a clear link with the devastation inflicted by the pandemic on sectors such as hospitality.
“It seems that much of the burden of job losses during the pandemic has fallen on non-UK workers and that has manifested itself in return migration, rather than unemployment,” the authors concluded.
“If this is even close to being accurate, this is the largest fall since World War II,” they added.
Brexit is not being pinpointed as a cause of the sharp decline, but could yet have implications for filling jobs when the economic recovery comes.
Under the harsher immigration rules, EU nationals would need work visas to return and work in Britain unless they have been granted ‘settled status’.
Furthermore, large changes in regional populations could make it more difficult for the NHS to distribute Covid-19 vaccines fairly around the UK.
The study examined the population change between the third quarter of 2019 and the same period in 2020, using labour data.
It rejected data published by the Office for National Statistics – finding a 500,000 drop, with employment of UK-born people in London rising – as “hardly plausible”.
The authors of the study, Michael O’Connor and Jonathan Portes, blamed those shortcomings on the difficulties in counting migration during the pandemic.
With officials unable to collect data in the usual way at airports and other transport hubs because of the pandemic, the ONS has acknowledged severe difficulties in measuring migration numbers.
Mr Portes, professor of economics at King’s College London, said Brexit might have had an impact “at the margin”, warning that effect might grow.
“The effects we describe here occur very specifically in 2020 Q1 to Q3, and are visible for both EU and non-EU migrants,” he tweeted.
But he added: “In 2021, interaction of Brexit, Covid-19, economic recovery, and new system is unpredictable.”