1. The recent Home Office estimate of unauthorised (illegal) migrants was four years out of date. It estimated the number to be between 310,000 and 570,000 with a central estimate of 430,000.
2. This estimate should be adjusted to include failed asylum seekers in subsequent years and the UK born children of illegal immigrants.
3. Taking these two factors into account Migration Watch estimates that the current population of unauthorised migrants is in the range 515,000 to 870,000 with a central estimate of 670,000. However, other factors point towards the upper end of this range.
Outline of Home Office's methodology
4. The Home Office published a report in June 2005 which attempts to estimate the size of the illegal migrant population in 2001 
5. The methodology used is to take the number of foreign-born people recorded in the 2001 census and compare this with the number of foreign-born people who are legally resident in the UK. The difference is the illegal migrant population. There is, incidentally, no reason why such a calculation could not have been made years ago.
6. Using this methodology the report's authors arrive at a central estimate of 430,000 illegal migrants and a likely range of between 310,000 and 570,000.
7. The foreign-born population who are legally resident in the UK consists of three elements:
Those granted settlement (i.e. indefinite leave to remain) in the UK.
Those who had an unexpired temporary leave to remain in the UK as at the date of the census.
Those who did not have an established reason to remain in the UK but who were awaiting a decision either on an asylum claim or on an appeal against a refusal of an application to stay. (quasi-legal migrants)
8. Totals for each of these three elements were derived from Home Office records and then adjusted for deaths and emigration.
9. For each of these elements the Home Office calculated a range of values (low, central and high). Before comparison with the census an addition was made to the census figures for the central and high estimates of 45,400 and 102,300 respectively to allow for a possible undercount of the foreign born in the census.
10. The lack of records of embarkation makes the sizing of the legal foreign-born population particularly difficult. Instead of actual records of emigration, the authors have had to deduce emigration rates from the International Passenger Survey (IPS) - a sample, voluntary questionnaire of people embarking from the UK. The exercise is therefore complex but, within the constraints imposed by weak immigration systems, every effort seems to have been made to come up with a reasonable estimate.
11. The one flaw in the report itself concerns children born in the UK who are not British citizens. The report says 
: 'Unmarried dependent children under 18 years of age who were born in the United Kingdom, but whose parents are neither British nor settled in the United Kingdom at the time of their birth, are not British citizens and are therefore subject to immigration control. Such a child requires leave to enter where admission to the UK is sought, and leave to remain where permission is sought for the child to be allowed to stay in the UK. In theory some of these children should be included under the 'unauthorised' population but children will normally be granted the same leave as their parents. So, if the parents continue to meet the Immigration Rules and remain in the UK, their dependent children will be granted leave in line. They are therefore not included separately in the calculations.' This is clearly wrong because it excludes UK born children of illegal immigrants.
12. Previous Migration Watch work has shown that the ratio of UK-born dependent children to the overall foreign born population is about 1 to 5 
. It could be argued that the illegal population are far more likely to be single than the foreign-born population generally. Migrants with families are also more likely to win appeals to allow them to stay in the country legally. Nevertheless, some allowance should be made for UK born children in the unauthorised population. We would suggest a range of 5% (low estimate), 10% (median estimate) and 15% (high estimate) of the unauthorised population. This would add 15,000, 43,000 and 85,000 to the reports estimates to give a range of 325,000 to 655,000 for the unauthorised population, with a mid-point of 473,000.
Subsequent developments (to end of March 2005)
13. The report's methodology is based on a census comparison so the conclusions are some four years out of date. The unauthorised population will undoubtedly have increased over this period for a number of reasons:
14. There has been a further four years of rejected asylum claims. Claims reached a peak (of 103,000 including dependants) in 2002 after the date at which the unauthorised population was sized.
15. There has been a reduction in the backlog of initial decisions and appeals on asylum cases which will have reduced the quasi-legal migrant population and increased the unauthorised population. (From 2002 to Q1 2005 inclusive 174,000 new asylum applications were lodged but 201,000 initial decisions were made. In the same timescale 136,000 appeals were lodged with the Home Office but 213,000 appeals were determined. 
16. There have been record levels of immigration to the UK in recent years increasing the risk of unauthorised overstayers. (In the 6 years from 1998 to 2003 net immigration from outside the EU to the UK has averaged just under 200,000 a year compared with 78,000 in the preceding 6 years 
17. There will have been continuing entry to the UK clandestinely and through false documentation. The number of persons against whom illegal entry action was initiated in recent years was 2000, 47,000; 2001, 70,000; 2002 48,000;2003, 23,000 (unofficial estimate due to poor data quality).
It is not known how many may have gained entry
These factors will be partially offset by:
18. The amnesty for asylum seekers with families announced by the Home Secretary in October 2003 which will have moved some migrants from the unauthorised category into legal migrants. It is thought that about 50,000 people will be given indefinite leave to remain as a result of this amnesty. However, most were still having their cases considered and would therefore have been counted in the quasi-legal category of legal migrants.
19. The accession of the 10 Eastern and Southern European countries to the EU in May 2005. This would also have had the effect of changing some people's status from 'unauthorised' to legal migrants. Up to 58,000 Accession country nationals who registered under the workers registration scheme may have been in the country before May,2005 
. However, many of these will probably have entered the UK after April 2001 and would not therefore have been counted in the illegal population at that time.
20. Since April 2001 there have been approximately 220,000 asylum applications which have failed but only 45,000 applicants have been recorded as being removed . Some may have left without being recorded 
. The ONS uses an estimate of 10% of rejected cases in preparing its international migration estimates. We consider that it is counter-intuitive to believe that such a significant number of people would leave of their own accord and undetected and we have therefore used a range of 10%, 5% and 0%. The recent National Audit Office (NAO) report on asylum removals (which confirmed the Migrationwatch estimate published on 15 April 2005) reported (page 13) only a trivial number detected - about ten in the month surveyed.
21. This gives an estimate of between 153,000 and 175,000 (central estimate 164,000) for the number of principal asylum claimants whose claims have failed but who have not been removed. We need to add about 25% to this to allow for dependants giving a total addition to the stock of failed asylum seekers of between 190,000 and 218,000 (central estimate 205,000).
22. The Home Office paper attempted to size the unauthorised migrant population thoroughly and objectively. But it should have included UK-born dependent children of unauthorised migrants and this would have increased the numbers of unauthorised migrants by 15,000 - 85,000. The central estimate would have been increased from 430,000 to 473,000.
23. The unauthorised migrant stock will have increased significantly since the effective date of the Home Office study (end of April 2001) by a rise of between 190,000 and 218,000 in the numbers of failed asylum seekers.
24. Accordingly, we estimate that the current unauthorised migrant population stock is, as the end of March 2005, in the range of 515,000 to 870,000 with a central estimate of roughly 670,000.
25. The offsetting factors - amnesty and Eastern Europe (para 14) are likely to be small. On the other hand, overstayers and clandestine entrants could be considerable (para 13). 1.5 million visas are issued every year; only a small proportion would need to overstay to affect the total considerably. Similarly, the number of clandestine entrants is unlikely to be less than - and could be a multiple of - those detected. The true total is therefore more likely to be at the higher end of the range.
28 July, 2005
||Home Office On Line Report 29/05 - Sizing the unauthorised (illegal) migrant population in the United Kingdom in 2001.
||Page 6 final paragraph of the above report.
||Migration Watch Research Paper 1.5 Contribution of Immigration to GDP at http://www.migrationwatchuk.co.uk/briefingPapers The data show that the foreign-born population of 4.896 million have 1.011 million dependent, UK-born, children
||Home Office Asylum Statistics 2002
||Home Office Asylum Statistics Q1 2005
||ONS International Migration Series MN28,29 and 30
||Hansard 17 March 2005 Col 395W
||Home Office Accesssion Monitoring Report May 2004 to March 2005 published on 26 May 2005.
||Home Office Asylum Statistics - Number of applicants who did not lodge an appeal following an adverse initial decision (difference between initial decisions resulting in a rejection and number of appeals lodged with the Home Office) and number of people whose appeals failed or were dismissed. Some who have had appeals rejected will have lodged further appeals but for the purposes of this analysis we have assumed that the number of applicants in this category will balance with those who had further appeals outstanding in 2001 and have subsequently exhausted the appeals process and had their claims dismissed.. All numbers for 2001 are assumed to be two-thirds of the full-year numbers to allow for the fact that the Home Office study was conducted at the end of April in line with the census.