Curbs designed to tackle illegal immigration are failing to reduce the number of people in the UK unlawfully, according to a new report.

A study found little evidence that immigration enforcement activity deters irregular migrants or encourages them to leave the country.

One interviewee told researchers that “being illegal in the UK is still better than being legal in my own country”.

Academics from the Centre on Migration, Policy and Society at the University of Oxford carried out hundreds of interviews for the project, titled Does Immigration Enforcement Matter?

Their paper concluded: “Our research provides little evidence suggesting that immigration enforcement brings down numbers of irregular immigrants.

“This research does however suggest that immigration enforcement has (unintended) side-effects, such as increasing human suffering whilst giving rise to criminal practices and pushing irregular immigrants further underground.”

Immigration Enforcement, which is part of the Home Office, is responsible for tracking immigration offenders and increasing compliance with immigration law.

Its stated “vision” is to reduce the size of the illegal population and the harm it causes.

But the new research said findings suggest immigration enforcement operations have “limited effects”.

Of 175 “irregular immigrants” interviewed, 40 had experienced enforcement operations, 11 were arrested and only one person was removed, according to the report.

Almost all interviewees were not deterred by tightened immigration controls or the “hostile environment” approach, the study found.

It said irregular immigrants have developed strategies to evade detection, most noticeably the use of false documents.

“This seems to be more common than 20 years ago and implies that tighter controls provoke more criminal responses,” the report added.

Lead researcher Dr Franck Duvell said: “One of the irregular immigrants we interviewed told us that ‘being illegal in the UK is still better than being legal in my own country’.

“This illustrates well why deterrence policies don’t work.”

He added: “Our project also shows that, if there is labour market demand whilst labour migration restrictions are in place, irregular migration is an inevitable consequence – which potentially means that efforts to restrict low-skilled EU migration after Brexit may generate a lot more irregular immigrants.”

The report warned that constant legal and institutional changes, as well as budget cuts, mean Immigration Enforcement is an agency under “substantial stress”.

Official statisticians insist it is not possible to accurately quantify the number of people in the UK unlawfully.

Last year a former immigration enforcement official claimed the figure was likely to run to more than a million.

In 2005 a Home Office assessment put the total unauthorised migrant population in the UK in 2001 at 430,000.

The “hostile environment” measures – which the Government now refers to under the heading “compliant environment” – have come under sharp focus following the Windrush scandal, which saw long-term UK residents challenged over their status despite living legally in the country for decades.

A Home Office spokeswoman said: “We are committed to an immigration policy which welcomes and celebrates people here legally, but which tackles illegal immigration, prevents abuse of benefits and services and removes incentives to enter and remain in the UK unlawfully.

“It’s what the public would expect as a matter of fairness, which is why it is right that we have a compliant environment to deter and tackle illegal immigration.

“Since 2010, Immigration Enforcement has protected public safety by removing more than 44,500 foreign offenders who have abused our hospitality by committing crimes and has disrupted over 180 immigration crime groups since April this year.”