It is difficult to measure how many people reside in the UK without authorisation, although a Home Office study based on Census 2001 data released in March 2005 estimated a population of between 310,000 and 570,000.^[1][2] The methods used to arrive at a figure are also much debated.^[3] Problems arise in particular from the very nature of the target population, which is hidden and mostly wants to remain so.^[4] The different definitions of 'illegality' adopted in the studies also pose a significant challenge to the comparability of the data. However, despite the methodological difficulties of estimating the number of people living in the UK without authorisation, the residual method has been widely adopted.^[5][6][2] This method subtracts the known number of authorised migrants from the total migrant population to arrive at a residual number which represents the de facto number of illegal migrants.^[7]

A study carried out by a research team at LSE for the Greater London Authority, published in 2009, estimated the illegal migrant population of the UK by updating the Home Office study.^[6] The LSE's study takes into account other factors not included in the previous estimate, namely the continued arrival of asylum seekers, the clearance of the asylum applications backlog, further illegal migrants entering and leaving the country, more migrants overstaying, and the regularisation of EU accession citizens.

The most significant change in this estimate is, however, the inclusion of children born in the UK to illegal immigrants. For the LSE team illegal migrants oscillate between 417,000 and 863,000, including a population of UK-born children ranging between 44,000 and 144,000. Drawing on this and taking stock of the outcome of the recent Case Resolution Programme,^[8] a University of Oxford study by Nando Sigona and Vanessa Hughes estimated at the end of 2011 a population of illegal migrant children of 120,000, with over half born in the UK to parents residing without legal immigration status.^[9] A Greater London Authority funded study by researchers at the University of Wolverhampton's Institute for Community Research and Development updated these figures in 2020, and estimated that the figure in April 2017 was between 594,000 and 745,000^[5] including between 191,000 and 241,000 children.^[5]

According to the House of Commons Library, several definitions for a migrant exist in United Kingdom so that a migrant can be:

• Someone whose country of birth is different to their country of residence.
• Someone whose nationality is different to their country of residence.
• Someone who changes their country of usual residence for a period of at least a year, so that the country of destination effectively becomes the country of usual residence.^[10]

Illegal immigrants in the UK include those who have:

• entered the UK without authority
• entered with false documents
• overstayed their visas
• worked or studied on a tourist visa/ non-immigrant visa waiver
• entered into forced or fraudulent marriage^[11][12][13]
• had their marriages terminated or annulled

Migration Watch UK, is a think-tank opposed to a large scale of immigration.^[14] Migration Watch UK has criticised the Home Office figures for not including the UK-born dependent children of illegal migrants. They suggested in 2007 that the Home Office had underestimated the numbers of illegal migrants by between 15,000 and 85,000.^[15]

Jack Dromey, Deputy General of the Transport and General Workers Union and Labour Party treasurer, suggested in May 2006 that there could be around 500,000 illegal workers. He called for a public debate on whether an amnesty should be considered.^[16] Former Home Secretary David Blunkett suggested that this might be done once the identity card scheme is rolled out.^[17] However the scheme was scrapped due to its widespread unpopularity by the coalition government in 2010.

London Citizens, a coalition of community organisations, is running a regularisation campaign called Strangers into Citizens, backed by figures including the leader of the Catholic Church in England and Wales, the Cardinal Cormac Murphy-O'Connor.^[18] Analysis by the Institute for Public Policy Research suggested that an amnesty would net the government up to £1.038 billion per year in fiscal revenue.^[19] However, analysis by MigrationWatch UK suggests that if the migrants granted amnesty were given access to healthcare and other benefits, the net cost to the exchequer would be £5.530 billion annually.^[20]

In February 2008, the government introduced new £10,000 fines for employers found to be employing illegal immigrants where there is negligence on the part of the employer, with unlimited fines or jail sentences for employers acting knowingly.^[21] In November 2008, the then Mayor of London, Boris Johnson, commissioned a study into a possible amnesty for illegal immigrants, citing larger tax gains within the London area which is considered to be home to the majority of the country's population of such immigrants.^[22] It was suggested in 2009 that to deport all of the irregular migrants from the UK would take 20 years and cost up to £12 billion.^[23]

In July 2013, the Home Office introduced an advertising lorry in London with its billboard saying "In the UK illegally? — GO HOME OR FACE ARREST — Text HOME to 78070 for free advice, and help with travel documents. We can help you return home voluntarily without fear of arrest or detention." This campaign was criticised by Vince Cable, a minister in the governing coalition, who called it "stupid and offensive".^[24] Nigel Farage of the UK Independence Party criticised the campaign as "nasty" and suggested that its real message was "Please don't vote UKIP, we're doing something".^[25]

In 2015 the newly elected Conservative Government announced it would be requiring Landlords to confirm the immigration status of tenants. Those failing to do so, or knowingly or unknowingly housing illegal immigrants could face criminal prosecution. This policy is called "Right to Rent", part of the Hostile Environment.^{[citation needed]}

In 2015 a large number of migrants had set up a camp at Calais in the hope of entering the UK illegally, although there have always been some migrants entering the UK illegally from here many illegal migrants or asylum seekers try to enter the UK from France, by hiding inside trucks or trains and they've done so for years.^[26] This sparked a large political debate in the UK. The UK government responded by funding additional security measures in Calais including a £7 million fence to prevent migrants entering the UK.^{[citation needed]}

There is significant public concern about illegal immigrants coming on small boats from France. The UN convention on refugees permits entry by an illegal route, despite the word 'illegal' implying a criminal offence has been committed.^{[citation needed]} Approximately 45,000 people arrived on small boats in the calendar year 2022.^[27] In the year ending March 2023, almost half of small boat arrivals were from two nationalities – Albanians (28%) and Afghans (20%)^[28]

On 18 July 2023, the United Nations criticised the UK's Illegal Migration Bill for violating international law and endangering refugees. The bill aims to curb boat arrivals and transfer migrants to countries like Rwanda, despite concerns over Rwanda's safety status.^[29] Simon Murray, the undersecretary of state for the Home Office, defended the bill, saying the nation's asylum system and budget were overwhelmed and that removing the possibility of staying in the UK is the only way to prevent smugglers from exploiting would-be migrants. Some individuals argue that allowing asylum seekers to make an application before travelling to the UK would make the people smugglers' services redundant, however this application would not result in the creation of alternative, legal routes not involving smugglers.^[30]

A report in 2020 found that almost 300 people had died crossing the English Channel on small boats in the twenty years prior.^[31] In October 2019 in a single incident, 39 migrants from Vietnam suffocated together in a lorry trailer travelling from Belgium to Essex. The Vietnamese leader of the smuggling gang was jailed in Belgium while a Romanian smuggler of the ring was sentenced at the Old Bailey.^[32][33]

• Demographics of the United Kingdom
• "Go Home" vans
• Immigration to the United Kingdom since 1922
• List of sovereign states and dependent territories by fertility rate
• MigrationWatch UK
• UK Visas and Immigration
• Calais Jungle
• Dunki a 2023 Indian film based on illegal immigration to the United Kingdom.