Civil servants have told ministers that attempts to respect the referendum result and reform the UK immigration system after Brexit could cause a “Windrush-style” crisis.
They claim that distinguishing between European Union (EU) migrants arriving after the divorce date and those who were here before, and therefore retain a right to remain and be treated as UK citizens, will be difficult.
Ministers hope to introduce a system from January 2021 that will end open borders and the ability of unlimited numbers of EU citizens to use the taxpayer-funded NHS and claim benefits.
They also hope to cut the number of low-skilled workers entering Britain and driving down wages. However, Mrs May has promised EU citizens already here they will retain the same rights.
Whitehall is trying to work out how to distinguish EU citizens with different rights after Brexit.
However, according to The Times, “officials” are making alarming warnings that a two-tier system will be “very difficult” to enforce in person, and online.
Existing migrants will retain the right of free movement, able to come and go, and the claim is that there is not currently a way to know at the border what EU migrants will still have this right.
Hospitals will also need to know when an EU migrant arrived in the UK to know if they can get free treatment and landlords have a legal responsibility to try and not let a property to people in the country illegally.
The Tory home secretary Sajid Javid wants to implement the system within two years, but sceptics say this is not feasible.
Last week Mr Javid reportedly told Cabinet ministers the UK’s borders would remain open to limitless EU migration for 30 months after Brexit.
According to his plan, the new arrivals will be free to live temporarily in Britain by simply showing their passport and passing a criminal check and will be able to apply for a visa, enabling them to stay permanently, in the two-year period.