Britain could have 300,000 more illegal immigrants than government figures suggest, a pressure group has claimed.
No past estimates of illegal immigrants have been published
Migrationwatch said Home Office figures of between 310,000 and 570,000 illegal immigrants were out of date and did not take into account migrants' children.
The Home Office said it was impossible to produce an accurate figure.
The Immigration Advisory Service lobby group said Migrationwatch had wrongly counted failed asylum seekers as illegal immigrants.
Migrationwatch said the government's figure, published in June this year, was based on 2001 census data.
The group claimed it had omitted record levels of immigration since then, including a peak in asylum claims in 2002.
Its report said the government had underestimated numbers of failed asylum seekers and other illegal immigrants by up to 218,000.
And it said there were between 15,000 and 87,000 children of illegal immigrants not taken into account - giving a figure of between 515,000 and 870,000.
Migrationwatch chairman Sir Andrew Green said the government's figures "rightly caused great consternation" because they showed the government had "lost control of our borders".
But Immigration Advisory Service chief executive Keith Best accused Migrationwatch of "idle speculation".
He told the Radio 4 Today programme: "Failed asylum seekers are not illegal immigrants. They are people who are subject to removal but have not yet been removed."
A Home Office spokesman said no government had been able to produce an accurate figure for the number of people living in the country illegally.
He said: "By its very nature it is impossible to quantify accurately and that remains the case.
"Our report makes specifically clear that it's producing one estimate using one possible methodology and cannot be seen as an accurate or definitive figure."
He added: "With regard to children, we think that there are some limitations in the calculations in our report, but we believe that this is the only sensible methodology which can be applied to the UK."