An in-depth study of the number of illegal immigrants in the UK should be launched, according to a National Statistics review for the Home Office.
There has only been one official estimate of illegal migrants
Last year, government researchers said the "best guess" was that there were between 310,000 and 570,000 illegal immigrants in the country.
But a new review of immigration figures calls for a more in-depth analysis.
Ex-government statisician Denis Allnutt, who authored the review, says such an analysis would be useful.
But he says the imprecise nature of the figures means there would be little point in issuing estimates routinely.
Mr Allnutt, a consultant working for the Home Office, says users of immigration figures find in-depth analyses useful.
Other areas he says could be tackled include: the characteristics of refugees; the number of failed asylum seekers in the UK; the number of asylum seekers supported by local councils; and the skills, special needs and languages of new immigrants.
Until last year ministers insisted it was impossible to produce figures for the number of illegal immigrants because of the very nature of the problem.
But last June a Home Office commissioned study produced an estimate by taking the numbers of foreign-born people living in the UK recorded in the 2001 Census and subtracting the estimated number of legal immigrants.
Since then a senior immigration official, and ministers, have said they do not know how many illegal immigrants there are in the UK.
The Home Office says it is working to improve the quality of its data as part of the review of the department and the Immigration and Nationality Directorate (IND).
"Work is in hand to identify strengths and gaps in the current quality of data and quality control," said a spokeswoman.
"Together this will underpin IND's overall information management strategy and inform the Home Office's future information system and technology needs."