The UK population may have fallen by as much as 1.3m - the biggest decline since the Second World War - in the wake of the Covid-19 pandemic, academics have said.

A study by the Economic Statistics Centre of Excellence highlighted an “unprecedented exodus” of foreign-born workers following the outbreak of the virus as well as shortcomings in official surveys inflating the number of UK workers.

Labour data - more reliable than migration statistics - suggest a drop of more than half a million in the number of foreign born residents in the year to last September, authors Michael O’Connor and Jonathan Portes said.

But they added that the Office for National Statistics had not adjusted population projections to take account of Covid-19 emigration, which exaggerated the number of UK workers in figures.

Their own estimates adjusting for emigration, which they called “a much more plausible picture of recent developments”, showed that instead of a rise of 350,000 in 2020, the total population “falls by more than 1.3m”.

“While the total population was almost flat in the late 1970s, if this is even close to being accurate, this is the largest fall in the UK resident population since World War 2,” the authors wrote.

The impact is felt largest in London, where the economists estimate the capital’s population may have fallen by almost 700,000 - echoing the 1970s exodus which saw central London’s population fall 20pc.

“Big shifts in population trends in London, driven by economic changes and events, are by no means historically unprecedented… so the picture of sustained growth driven by international migration is relatively recent. If this has now reversed, the medium to long-term implications for London will be profound,” the report said.

The adjusted population estimates also helped explain why official unemployment remains so low at 4.9pc despite the economic shock of Covid.

“Much of the burden of job losses during the pandemic has fallen on non-UK workers and has manifested itself in return migration, rather than unemployment”, they added.

Falling and ageing populations pose a long-term threat to the economy's ability to grow. Covid-19 inspired emigration is not the only threat to the UK’s population according to recent research, as the nation also faces a “baby bust” this year.

The impact of the lockdown and the economic fall-out of the virus on couples delaying starting a family is likely to push the birth rate to its lowest on record, according to estimates from accounts PWC.

Its economists suggested that just 569,000 babies are likely to be born this year, down from as many as 724,000 in 2011, and the lowest figure since 1900.