Net long-term migration to the UK fell below a quarter of a million in 2016 for the first time in nearly three years as arrivals from eastern and central European countries plunged dramatically.
The overall measure - the difference between the number of people arriving and leaving the country - was estimated to be 248,000 last year.
This was a "statistically significant" fall of 84,000 compared with the figure recorded in 2015, the Office for National Statistics (ONS) said.
Statisticians said the change in long-term international net migration - covering people coming to and leaving the country for at least 12 months - was driven by a rise in emigration, which was up 40,000 on 2015.
This was made up mainly of EU citizens, with the number departing at an estimated 117,000 - a rise of 31,000 on the previous 12 months.
Immigration - just those arriving - was at 588,000 last year, a fall of 43,000 year on year, although the ONS said this was not a statistically significant decrease.
The figures - the last official migration data before next month's election - show citizens from eight central and eastern European nations have partly driven the changes.
Immigration from EU8 states - the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Slovakia and Slovenia - was down by 25,000 to 48,000, while emigration increased by 16,000 to 43,000 last year.
These were both statistically significant changes and resulted in the smallest net migration estimate, of 5,000, for the group of nations since they joined the EU in 2004.
The last time overall net migration, which covers the EU and the rest of the world, was below 250,000 was in the year to March 2014.
The Conservatives have maintained their objective of reducing annual net migration to the tens of thousands ahead of the poll on June 8.
In its election manifesto, Labour said it was offering "fair rules and reasonable management of migration" - but dismissed targets as "bogus".
The new figures give the fullest picture yet of immigration around the EU referendum vote last year.
Britain's settlement following departure from the bloc is expected to include restrictions on free movement rules, although the precise arrangements for post-Brexit immigration are yet to be thrashed out.
The official figures also showed that:
:: Net migration from the whole of the EU was 133,000 in 2016, down 28% on 2015;
:: There was a statistically significant increase in the estimated number of non-British citizens going home to live, largely accounted for by EU nationals;
:: Non-EU net migration was estimated to be 175,000 last year, a fall of 14,000 compared with the previous year;
:: Work remains the most common reason for international migration, with 275,000 people immigrating for employment, the majority of whom had a definite job (180,000);
:: Long-term immigration to study saw a statistically significant annual fall of 32,000 to 136,000.
The ONS report said: "Long-term immigration of EU citizens rose from 2012, levelled off from 2015, and has seen a slight decline in the last two quarters. It is too early to tell if this trend will continue."
Dr Carlos Vargas-Silva, acting director of the Migration Observatory at the University of Oxford, said: "While public opinion in the UK has supported reductions in immigration for many years, achieving these sorts of cuts is difficult in practice.
"It seems unlikely that we will see net migration in the tens of thousands in the near future without either an economic downturn, or a new set of much more restrictive immigration policies.
"Even with the current low levels of net migration from the EU8 countries, the total estimate is still more than twice the Government's target."
Saira Grant, chief executive of the Joint Council for the Welfare of Immigrants, said: "Many EU citizens feel unwanted and insecure and are leaving the country without even attempting to apply for residence.
"After the General Election, the Government must show strong leadership by guaranteeing the rights of EU citizens currently in the UK.
"These Europeans are our friends, neighbours and family members. They are an integrated and welcome part of our society."
Home Secretary Amber Rudd told the BBC: "We are determined to make sure that we do continue to reduce the overall net migration number, but also continue to attract the brightest and the best and support our economy."
Alp Mehmet, vice chairman of Migration Watch UK, a think tank which campaigns for tighter immigration controls, said: "This reduction in net migration is welcome but it is still running at a quarter of a million a year - a level that would once have been dismissed as incredible."