Thousands of people are faking living in Ireland to get family members into the UK, a BBC investigation has revealed.
The scam involves UK nationals who want to bring in close relatives from outside the European Economic Area.
Police said immigration advisers, lawyers and accountants were behind the multi-million pound fraud.
The UK government is warning it is a growing industry that exploits European free-movement rules.
The scam uses the so-called Surinder Singh route, named after a historical immigration court case.
If a UK national lives and works in another European country for a period of time, they can be considered under EU rather than British law on their return.
That means that if they have been joined by a non-EEA spouse, they are allowed to bring them into the UK without having to meet certain immigration requirements that apply to Britons.
This route has grown in popularity since 2012, when the government introduced a minimum income a UK citizen had to earn before they could bring a spouse from outside the EU into the UK.
Each year, about 20,000 non-European family members come into the UK this way.
The fraudsters are charging thousands of pounds to create a fake life so it looks like someone has genuinely moved to a European country – in most cases Ireland – while in fact they have stayed in the UK.
Det Supt Stephen Courage, from the Garda National Immigration Bureau, said fraudsters were charging up to £25,000 for the scam.
“The facilitator will quite often set up a company, of which you will either be an owner or a director,” he said.
“They will also create a work history for you.
“They will create payslips, they will open bank accounts, and also pay nominal tax so when the immigration service receives an application to exercise EU treaty rights, they will look at the paperwork and on the face of it, it will appear that you have a life in Ireland.
“The people we are coming across in our investigations are often from a professional background, whether it be in law or accounting, these are white-collar criminals.
“The profits they’re making are staggering. We would see this as a multi-million euro enterprise.”
Det Supt Courage said police were examining cases from the past three years and had already identified 600 where they believed someone had obtained EU treaty rights illegally.
UK Immigration Minister Robert Goodwill said: “There is a growing industry, fed by unscrupulous immigration agents, that seeks to exploit free movement rules to help non-EEA nationals circumvent our immigration system, creating backdoor routes into the UK.
“Last month we introduced tough new regulations which allow us to remove these individuals and ban them from re-entering the UK for 10 years.”
