The National Audit Office (NAO) said that the latest estimate in 2005 is that there are around 430,000 people in the country who are not allowed to stay.
But independent research has since put the figure more than a million dollars, Whitehall of watchdog spending said.
The NAO’s report in the Home Office of Immigration, enforcement branch, said the ministry estimated the demand for immigration officials from the activity.
This has been put at between 240,000 and 320,000 cases per year.
But the report said there was no database against which progress could be measured or to demonstrate that demand was up or down.
The watchdog concluded that despite gathering information around its mission and goals, the Home Office is often unable to demonstrate that its measures are working.
In the year to October 2019, detected attempts by people to come to the UK by clandestine means increased by 46,900.
However, the NAO said it was unclear if it was because more attempts were made or if there was better detection.
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His report added that the Home Office is unable to assess whether measures to prevent people from accessing government-funded services have a significant impact on the likelihood that an individual will voluntarily leave the United Kingdom “.
The watchdog said that the Home Office could “improve its understanding of the full scale of the illegal population in the UK and identify new ways to address some of the challenges it faces” by looking how other parts of the administration produce estimates of “hidden activity”.
A Pew Research Center poll estimated last year that the UK’s illegal immigrant population was $ 1.2 million, but the NAO said it hadn’t attempted to verify this. figure.
“Even if there would be great uncertainty around an estimate [the department] developed, such an estimate could help the department demonstrate that its activities are effective in deterring attempts to enter or stay in the UK illegally, “said the report.
Meg Hillier, chair of the House of Commons Public Accounts Committee, said the analysis demonstrated that the Home Office “has no idea how many people are in the country illegally and does not seem interested in finding ”
The Labor MP, added, “He cannot demonstrate that his illegal immigration controls are working as intended, and fail to understand how the different aspects of his work fit together. ”
NAO head of Gareth Davies said that immigration officials was “by nature” a “complex and difficult” task.
“While the Home Office has introduced significant changes to its monitoring activity, it cannot demonstrate that overall performance is improving,” he added.
“The ministry needs a better understanding of the impact of its immigration agents of activity on its global vision to reduce the size of the illegal population and the damage it causes. ”
A Home Office spokesperson said, “We have regained control of our immigration system and for the first time in a generation, we will have full control over who comes and stays here.
“As this report recognizes, the nature of immigration of crime and delinquency is complicated and we are constantly looking for ways to get organized gangs behind it in advance.
“We are working tirelessly with international partners and organizations, such as the ANC, to combat illegal migration, close the illegal immigration routes and the return of these with no right to stay in the UK in the whenever possible.
“We make no apologies for anyone seeking to deport foreign offenders who and since 2010, we have abducted over 53,000 criminals. “