By Dominic Casciani
Almost one in 10 British citizens is living overseas, according to a study of people coming in and out of the UK.
Studies by the Institute for Public Policy Research, published on the BBC News website, indicate that at least 5.5m British-born people live abroad.
Figures suggest the rate of departure has been so great that population falls are only masked by immigration.
While Australia and Spain are the top locations, increasing numbers are heading to major Asian economies.
The research by the institute (IPPR) is the first significant attempt to put a figure on the number of British citizens who live overseas.
The study found 5.5 million expat Britons - a number that rises to six million if those who live or work part of the year abroad are included. Taken together, they represent approximately 10% of British citizens. Until now, tentative government estimates of expats had ranged from 4.5 million to 14 million.
Over the course of 40 years, some 67,500 more Britons have left the UK every year than have returned - a population loss that has been balanced out by increasing immigration.
The number of British citizens who chose to go permanently abroad doubled from 53,000 in 2001 to 107,000 last year - some 2,000 people a week.
The majority of expats live in Australia, Spain, the US and other English speaking nations. But in all some 41 nations each have at least 10,000 permanent British residents. The IPPR's figures have been calculated from official sources, such as census data, counts of pensions paid overseas, passport applications and other statistics.
Young and working
According to the IPPR's research, those most likely to leave the UK are young workers without families, along with those seeking to retire.
Who left in 2004?
Source: International Passenger Survey/ONS
Dhananjayan Sriskandarajah, co-author of the report, said that the scale and spread of the British expat diaspora was probably being driven by the UK's economic strength.
A strong economy was attracting economic immigrants - but also encouraging Brits to broaden their opportunities.
"Britain is truly at the crossroads of the global movement of people," said Dr Sriskandarajah.
"Two-thirds of Britons who leave do so to seek employment abroad - and are replaced by skilled professionals from elsewhere in the world.
"When the going is good, Brits get going."
More emigration expected
Earlier this year, a BBC survey on emigration attitudes found the number hoping to leave in the near future had doubled since 2003.
Young people were the most likely to want to leave, with a quarter saying they were hoping to live abroad.
Dr Sriskandarajah said the new figures indicated emigration would grow further.
"If current trends continue, we could expect as many as a million more British nationals to emigrate over the next five years," he said.
But Sir Andrew Green of Migrationwatch UK said the departure of British citizens should be seen against an annual arrival of some 300,000 immigrants.
"This net migration is the key issue - that is those who come minus those who go," he said.
"We now face by far the largest wave of immigration since 1066, even allowing for those who emigrate. This is putting enormous strain on our infrastructure, public services and on social cohesion."
Lord Triesman, Foreign Office Minister for Consular Affairs, said he welcomed the report.
"Globalisation has increased movement of people both to and from the UK," he said.
"The policy challenges are how to manage these flows effectively in order to respond to the changing needs of the UK. This research will be helpful in understanding how best to address those challenges.
"The report also confirms what we at the FCO know to be true - that a small but significant proportion of British emigrants run into trouble because they have not planned their departure properly.
"Much of what we do is designed to help people avoid such pitfalls, and we will continue to work hard in this area. "
Foreign Office survey
The Foreign Office is researching consular needs and passport applications. It is seeking the views of British citizens in Hong Kong, the USA, Spain, Germany, Barbados, Saudi Arabia, Indonesia and Nigeria. To take part, click on the link to go the survey website run by market research company TNS.