The uk government relies on a disturbingly weak evidence base in assessing the effects of its immigration enforcement and has little knowledge of basics such as how many undocumented migrants are in the country, parliaments public spending watchdog has said.

The house of commons public accounts committee made the findings in a scathing report scrutinising the effectiveness of the home offices 392m annual spending on immigration enforcement. the department says the aim is to reduce the number of people living in the uk without permission and the harm that population causes.

However, the committee found the department had not estimated the size of the undocumented population since 2005, when it was put at 430,000. the independent pew research center of the us last year estimated that between 800,000 and 1.2m people were living in the uk without permission.

The department could not respond to our concerns that potentially exaggerated figures calculated by others could inflame hostility towards immigrants, the report said. it currently estimates that between 240,000 and 320,000 people per year come into contact with its immigration enforcement services, but also recognises that the quality of its information is not good enough to provide a baseline to measure progress against its vision.

The governments desire to reduce the number of undocumented people living in the uk has been a key driver of home office policy in recent years. the goal has led to the steady escalation of hostile environment policies intended to push undocumented migrants to leave the uk, making it impossible for them to earn money, rent property or open a bank account.

The policy has been criticised for causing harassment and mistreatment of many people legally resident in the uk, including members of the windrush generation of migrants from commonwealth countries who arrived before 1973.

The committee said the departments disturbingly weak base of evidence about the nature of the undocumented population meant it had little sense of whether its policies were achieving their stated aim.

It allocates resources between its different immigration responsibilities based on judgments, but if those judgments are not based on evidence, it is unclear what factors the department is considering, the report said.

The committee was particularly critical of the home offices failure to understand the nature of the mistakes it was making including the fact that it was forced last year through legal or other action to release 62 per cent of people it had intended to remove from the uk.

The obstacles to removing would-be asylum seekers from the uk have become clear this year as the department has faced numerous legal challenges, many successful, to its efforts to send migrants in the uk back to countries such as spain, where they first entered the eu.

The department believes this rise reflects abuse of asylum claims and other protection routes, but it did not provide any systematic analysis to support this, the committee said. given the strong passions seen on all sides of the immigration debate, a government department making unsupported claims of this kind risks inflaming prejudices against legitimate immigrants and bona fide asylum seekers.

The report said that, instead of working on the basis of sound information, the department risked making decisions on the basis ofanecdote, assumption and prejudice.

One home office official said the home secretary, priti patel, agreed withthe committee's assessment of historical issues at the department.

Referring to aninquirythat was highly critical of the departments handling of issues relating to the windrush scandal, the official said: she has spoken at great length about how the department puts process before people and this is why she has committed to implementing the findings of the wendy williams review into windrush.