Up to half a million refugees and their relatives could move to Britain after 2020 because of EU rules on the free movement of people, a major new report claims.
While Germany, Greece and Italy have borne the brunt of the refugee crisis arriving at Europe's borders, those granted asylum could settle in the UK in the coming years after they have acquired EU citizenship.
A report by Migration Watch, which campaigns for tighter border controls, said an analysis of EU figures show that hundreds of thousands of people could take this route into the UK.
Lord Green of Deddington, chairman of Migration Watch UK, said: "The UK could well face a significant secondary flow of refugees from Europe in the coming years adding to the already huge strain being placed on housing and public services.
"While the UK has so far been largely shielded from the crisis in southern Europe, this potential flow can only add to the impact of migration which is already seriously affecting communities across the country."
The report, entitled The Refugee and Migrant Crisis in the EU, says that according to Eurostat a projected 968,000 migrants will have been granted asylum or humanitarian protection for 2015 and the first quarter of 2016.
According to leaked estimates from the German authorities, each person granted asylum is likely to be followed by between four and eight family members.
Migration Watch said that taking the lower figure of four would suggest the total inflow to Europe could amount to 4.8 million over a period of years - without taking account of any future arrivals who might successfully claim asylum.
The report said that if 10% of these refugees decided to move to Britain once they got EU citizenship, which can be claimed after around five years, then 480,000 people could arrive in the UK in the years following 2020.
Lord Green told the Press Association: "Yes, we were surprised at the figure, especially as we only focused on those refugees who have already arrived in Europe.
"But it is important to be clear that this would be spread over several years, it would be from 2020 onwards, they are not going to come in a single movement."
The report said these refugees may be attracted to Britain because they have relatives here, know the language or think they will have more job opportunities in the UK.
The report said research showed that between a third and half of all Somalis granted refugee status in the Netherlands have moved to the UK.
Lord Green said: "The people we are studying here will be granted EU citizenship after a period of four to six years depending on where they are.
"And as EU citizens they will then have a right to enter the UK as and when they wish so that is why we won't be able to control that further movement.
"We have found over some years that there has been very little forward thinking about the scale of immigration and what needs to be done to meet the needs of migrants when they get here. It is fair to say this is pointing ahead and hoping that this prospect will form part of Government's planning."
Former Conservative Party leader and pro-Brexit campaigner Iain Duncan Smith said the report highlighted how little control Britain has over is borders while it remains in the EU.
He said: "This report brings yet more evidence of the uncontrollable scale of immigration to this country.
"The Remain campaign may try to tell you that immigration and the economy are separate, but the truth is that they are fundamentally linked.
"Immigration is an economic issue because it has had a big impact on people's wages - forcing them down, even as the cost of living has risen. It has also had a dramatic impact on people's ability to afford their own homes, and their ability to access good public services."
He said it is Britain's poor that feel the adverse consequences of immigration.
He said: "Big banks and highly paid corporate executives may fail to realise the link - because they have the means to afford alternatives, As always, when public services are under pressure it is those without those same resources who suffer.
'With four million people coming to this country over the last decade - and four million more due to arrive in the next - there will be serious consequences for the British people.
"The truth is that we can do nothing about this for as long as we remain in the European Union."
The Stronger In campaign, which wants Britain to stay in the EU, branded the report "completely false and bogus" and said its numbers are "completely wrong".
It also questioned Migration Watch's independence, accusing the organisation of having "close links with Leave campaigners".
Emma Reynolds, Labour MP for Wolverhampton North East, said: "This false report cannot hide the fact that leaving Europe would wreck the UK economy.
"The evidence shows the overwhelming majority of refugees will never get the right to come to Britain.
"There is only one thing we know for certain – leaving Europe will wreck our economy.
"That will cost jobs, damage people's livelihoods, and weaken our economic future. Our children and grandchildren will pay the price."