A scheme to allow EU citizens to remain in the UK after Brexit has exposed a failure to keep an accurate track of the number of migrants in the country.
More than 5.6 million people have applied for settled status, which allows EU passport holders to continue to live, work and study in Britain. The settled status scheme closes next week.
That number far outstrips an official estimate from March 2019 that there were 3.7 million EU citizens in the country.
The Office for National Statistics has long acknowledged that its system for recording migration, largely based on a survey of arriving passengers, was “stretched beyond its original purpose” and has developed new methods to provide more accurate figures.
The new technique, based in part on benefits and tax data, reveals that the UK underestimated net migration from the EU by 839,000 over nine years from 2012 to 2020, ONS figures show.
Immigration is a hot political topic in the UK, with opposition to the number of migrants arriving in the country seen as a significant factor in the 2016 Brexit referendum .
In 2010 former prime minister David Cameron in an appeal to voters promised to reduce annual net migration to the tens of thousands.
Even with migration sharply down because of the coronavirus pandemic, official figures on Friday showed that 620,000 people migrated to the UK from the rest of the world in the year to mid-2020. About 375,000 left the country.
The rise came during the lowest population growth period in the UK for nearly 20 years – just 284,000 to 67.1 million – largely because of the high number of deaths from Covid-19 but also a result of low birth rates.
Neil Park, of the ONS population estimates unit, said: "The 12 months to June 2020 can be broken into two clear parts: the first eight months, when births, deaths and migration patterns were similar to trends seen in recent years; and the four months from March, when the first wave of coronavirus hit.”