The government has not had an up-to-date estimate of the number of immigrants in the UK illegally in 15 years, according to a report.
The last official estimate in 2005 found about 430,000 people were in the country with no legal right to remain.
But independent research since then has put the figure at over a million, the National Audit Office said.
The Home Office said it worked “tirelessly” with other departments and partners to tackle illegal migration.
Meg Hillier, from Parliament’s financial watchdog, said it appeared the Home Office had “no idea” how many people were in the UK illegally.
The Labour MP, who chairs the public accounts committee, added the department “doesn’t seem interested in finding out”.
The report found that the Home Office put demand for its immigration enforcement services at between 240,000 and 320,000 cases per year.
But the NAO said this figure excluded those who had no contact with government in the previous two years, and “does not yet provide a baseline” to measure the effectiveness of the department’s enforcement action.
“Due to data quality issues, Immigration Enforcement cannot say whether the number of people ‘genuinely putting a demand’ on its activities is increasing or decreasing,” it found.
It suggested the Home Office should look at how other parts of government come up with estimates of “hidden activity” such as serious and organised crime, which it said could help to “improve its understanding of the full scale of the illegal population in the UK”.
The highest estimate of the illegal population in the UK is 1.2m, given by the Pew Research Centre last year – although the NAO said it had not attempted to verify the number.
The NAO also found that voluntary and enforced returns of people living the UK illegally had “fallen dramatically” since 2015.
Voluntary returns had reduced from an average of 1,200 a month in 2015 to approximately 460 a month in 2019, the watchdog said – although the number of foreign criminals departed had “remained more stable”.
In their 2019 election manifesto, the Conservative Party pledged to “keep track of who is coming in and out of our country”.
Garth Davies, the head of the NAO, said: “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.”
A spokesman from the Home Office said it was working “tirelessly… to tackle illegal migration, close down routes for people smuggling and return those with no right to remain the UK wherever possible”.
He added: “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.”