Britain could have as many as 570,000 immigrants staying in the country without permission, the Home Office said in a study on Thursday. The report, based on 2001 census data, is the government's first attempt to put a figure on the scale of illegal migration.
It calculates that the number of unauthorised immigrants ranges from 310,000 to 570,000, with a central estimate of 430,000 or 0.7 percent of the population.
The Home Office stressed the figures were a "guesstimate".
"No government has ever been able to produce an accurate figure for the number of people who may be in the country illegally," a Home Office spokeswoman said.
The report follows government-commissioned research into the best way of calculating the level of unauthorised immigration.
The estimates were reached using an American system, called the Residual Method, which compares estimates for legal migration with census data for the foreign-born population.
The figures do not include asylum seekers whose applications are being processed or who are appealing against deportation.
Immigration remains a hot political issue in Britain, despite falling numbers seeking to stay in the country.
The government is promoting its controversial identity card plans as one way of tackling illegal entry to the UK. The number of people seeking asylum in Britain fell 17 percent to 7,000 applications in the first three months of 2005, with Iranians, Iraqis and Somalis heading the applicants.
Asylum applications peaked at over 80,000 in 2002, falling to 33,000 last year. The fall in applications prompted the government earlier this month to abandon controversial plans to build holding centres for asylum seekers across the country.