Because residency in the UK is based on European free movement laws, rather than British immigration rules, anyone who appears to have worked at another European Economic Area (EEA) state is free to move to another.
This means anyone who has lived and worked in the Republic of Ireland can move across the Irish Sea to the United Kingdom.
Fraudsters are now making millions by creating a fictional history for migrants from outside the EEA.
Migrants create 'fake lives in Republic of Ireland' to cheat their way into UK
With their new, false EEA histories and employment and residency in Ireland, these migrants are then moving without obstruction to the UK.
We would see this as a multi-million euro enterprise
Stephen Courage – Garda National Immigration Bureau
Irish police warned the scam was staggering in its complexity and scale, with 600 cases already flagged up as suspect.
He also highlighted the huge profits being made by those organising the scams, with fraudsters pocketing up to £25,000 per false life in Ireland created.
Det Supt Stephen Courage of the Garda National Immigration Bureau said: "The facilitator will quite often set up a company, of which you will either be an owner or a director.
Anyone who appears to have worked at another EEA state is free to move to another
"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.
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"The profits they're making are staggering. We would see this as a multi-million euro enterprise."
Irish police warned the scam was staggering in its complexity and scale
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UK Immigration Minister Robert Goodwill said the UK government was doing everything it could to crack down on the scam.
He 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."