The Home Office has not made an up-to-date estimate of the size of the illegal population in the UK in 15 years, according to a report.
There were roughly 430,000 people in the country with no right to remain here when the last estimate was made in 2005 – but independent research since then has put the figure at more than 1 million, the National Audit Office (NAO) said.
Its report into the Home Office's immigration enforcement directorate said the department has instead estimated demand for immigration enforcement activity, which it put at between 240,000 and 320,000 cases per year.
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But there is no baseline given against which progress can be assessed or to show whether demand for enforcement activity is going up or down, the report added.
The highest estimate of the illegal population in the UK is 1.2 million, given by the Pew Research Centre last year, but the NAO acknowledged it had not attempted to verify the number.
The report said: "Although there would be significant uncertainty around any estimate it [the department] developed, such an estimate could help the department to demonstrate that its activities are effective in deterring attempts to enter or remain in the UK illegally."
Labour MP Meg Hillier, head of Parliament's financial watchdog, said it appeared the Home Office had "no idea" how many people are in the UK illegally "and doesn't seem interested in finding out".
She added: "It can't demonstrate that its actions to control illegal immigration are working as intended, and doesn't understand how different aspects of its work fit together."
The NAO's report also concluded that despite collecting information around its missions and objectives, the department often cannot show whether its measures are working.
While detected attempts by people to come into the UK by clandestine means has risen to 46,900 in the year to October 2019, from 40,800 in the same period for the previous year, it is unclear whether that was down to more attempts being made overall or better detection, the report said.
The department is also unable to assess whether its measures to prevent people accessing government-funded services "have any meaningful impact on the likelihood that an individual will leave the UK voluntarily", the report added.
Gareth Davies, head of the NAO, said: "The work of immigration enforcement by its very nature is complex and challenging.
"While the Home Office has introduced significant changes to its enforcement activity, it cannot demonstrate that overall performance is improving.
"The department needs a better understanding of the impact of its immigration enforcement activity on its overarching vision to reduce the size of the illegal population and the harm it causes."
During last year's general election campaign, home secretary Priti Patel vowed to "reduce immigration overall".
A Home Office spokesman said: "We have taken back 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 acknowledges, the nature of immigration crime and offending is complicated and we are consistently looking at ways to get ahead of the organised gangs behind it.
"We work tirelessly with international partners and agencies, such as the NCA, to tackle illegal migration, close down routes for people smuggling and return those with no right to remain the UK wherever possible.
"We make no apology for seeking to deport foreign national offenders and since 2010 we have removed more than 53,000 criminals."