Research from the population pressure group Migration Watch estimated that accepting a Brussels demand for unlimited free movement to continue for a transitional period of up two year after Brexit could lead to a significant increase in the numbers allowed to come into the country before new border controls are imposed.
A draft document leaked from Brussels earlier this week showed that EU negotiators will make the demand in the next phase of talks about the UK’s departure from the bloc.
They want EU citizens to continue to be able to claims full permanent residency rights in Britain throughout the transition period.
The status would give European migrants the right to be joined by direct family members, including elderly dependent relatives, future children and partners who were not EU citizens.
Migration Watch urged the Prime Minister to firmly resist the demand.
Lord Green of Deddington, chairman of Migration Watch, said: “This EU demand is absolutely absurd.
“We will have left the EU – that must mean that EU citizens who arrive after our departure should no longer enjoy treaty rights that derive from our membership.”
Migration Watch also estimated that a three-year transition period, wanted by some Brussels officials, could increase the number of EU citizens with permanent residency rights by 1.5million.
The research was based on analysis of past migration data.
The figure of half a million a year is based on the number of EU citizens who, in recent years, have applied at a Job Centre for a National Insurance number.
In the past, some of the applicants have been temporary migrants who stayed for less than a year but a larger proportion of them might well stay on if the prize was a right to permanent settlement in the UK.