A new study has found that strict Government controls on immigration drive up the number of people entering a country illegally.
Academics believe that while visa restrictions cut overall migration, they can be ineffective and counterproductive because they force more people to find unlawful ways to cross borders.
The study also found that restricting the number of visas granted to students and highly-skilled workers does little to cut overall migration.
Tighter restrictions on low skilled workers do appear to have reduced migration but it has led to a significant portion of prospective immigrants turning to illegal ways to enter a country.
The study published in Proceedings of the National Academy of Sciences, was conducted by academics at the University College London (UCL) in partnership with colleagues at the Royal Holloway and University of Birmingham.
Ministers have come under pressure to ease strict visa restrictions and abandon their promise to cut net migration to the “tens of thousands”
Theresa May has already agreed to lift restrictions on the number of doctors and nurses allowed to enter the UK each year after sustained lobbying from NHS leaders and some of her own cabinet ministers.
The new study found that restrictions on family migration were especially unhelpful, leading around a quarter of people who would have moved country legally to instead do so via illegal channels.
Co-author Professor David Hudson, from the University of Birmingham, said: “Our research clearly demonstrates that, while restrictions on immigration do decrease legal migration, this has unintentionally led to an increase in illegal immigration flows which results in a costly and unsustainable need for greater border enforcement.