IT costs £390 million a year and is supposed to keep illegal migrants out of the UK – but the Home Office can’t prove its Immigration Enforcement directorate works, a watchdog says.

Like Tory governments before it, Boris Johnson’s administration has tightened immigration rules in a bid to drive down the number of those settling in the UK.

But the last time the Home Office worked out an estimate for the number of people here without legal permission was 15 years ago.

And more than 60% of those taken into detention for deportation are released without removal.

As The National has highlighted, many of these cases involve people who later win the right to remain in the UK – such as Chennan Fei, who was brought to Scotland from China as a child by her parents, who failed to ensure her paperwork was kept up to date, and Kenyan Christian Robert Makutsa, whose relationship was said to be fake because he would not live with his partner before marriage.

They also include Vietnamese modern slavery victim Duc Nguyen, whose fight for refugee status took more than two years.

Cumbernauld mum Olya Merry was told she’d be detained and sent back to Belarus if she didn’t leave herself within seven days in 2018.

And the Brain family of Dingwall were also told they could be forcibly repatriated to Australia in 2016 after falling foul of changes in the visa system that happened as they flew to Scotland.

Campaigners say the system is cruel, harmful and does not work.

Now the National Audit Office (NAO) says the Home Office “cannot fully demonstrate that the actions of its Immigration Enforcement directorate” are doing what they’re supposed to.

A report published today states that while the Home Office estimates demand for immigration enforcement activity to be between 240,000 and 320,000 cases per year, it “does not know whether demand is growing or falling” and has not consistently defined what constitutes “harm” caused by the illegal population and who this affects.

The watchdog found Priti Patel’s department is also “unable to assess whether its measures to prevent people from accessing government-funded services have any meaningful impact on how likely a person is to return to their country of origin”.

Fewer than half of planned enforced returns actually went through last year, with 14,900 people released from detention centres.

More than 13,100 people without leave to remain were removed from the UK un the 12 months to the end of November 2019. Around 5600 of these were classed as voluntary returns.

Around 5000 of those removed were foreign national offenders.

The NAO highlighted “inefficiencies” in Immigration Enforcement processes which leave individuals waiting longer for a result in

their cases.

But these delays are not systematically recorded to find out how and where improvements can be made

The findings come as the UK Government considers its response to the Windrush review.

A review of the wider border and immigration system is also under way. The NAO says these will give the Home Office the chance to address many of the issues it has identified, but it says the department must develop a “better understanding” of the effects of its work.

NAO head Gareth Davies 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.”

Meg Hillier MP, chair of the Public Accounts Committee, said the Home Office “doesn’t understand how different aspects of its work fit together.”