GETTY A soft Brexit could see 12 million people enter the UK over two decades

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Figures showed that annual net migration to the UK from EU countries would be unlikely to fall below 155,000 in the "medium term" if the country stays in the EU's Single Market. And the Government's ability to reduce the influx will be "extremely limited" as long as the EU's free movement rules remain in force. The disturbing forecast of near-record levels of migration for years ahead was set out in a report from the population think tank Migration Watch. It was being seen last night as a devastating blow to campaigners for a so-called "soft Brexit" that retain the UK's full Single Market membership.

Supporters of a full break with Brussels insisted that snatching back control over immigration was at the heart of the vote to leave the EU in last summer's referendum on the UK's membership of the bloc.

An increase of anything like 12 million in just 25 years is, quite simply, unacceptable to the British public Alp Mehmet, Migration Watch UK

Alp Mehmet, vice-chairman of Migration Watch UK, said: “This research spells out the very serious consequences for our society of net migration continuing at its present scale with membership of the single market resulting in a relentless increase in our population. "An increase of anything like 12 million in just 25 years is, quite simply, unacceptable to the British public and certainly not what they voted for in the referendum.” Tory MP Philip Hollobone said: "Migration Watch's forecasts have always been thoroughly reliable and are the best we are going to get from anyone.

GETTY If EU laws on free movement remain, the Government will be limited with its migration options

"The picture they paint is completely unacceptable. "We simply can't stay in the Single Market if it means we are going to get that sort of level of immigration. "Above all, people voted in the referendum for a change in our approach to immigration." And John Bickley, Ukip's immigration spokesman, said: "Migration Watch have confirmed Ukip's worst fears, that remaining in the Single Market will not result in the UK 'leaving the EU' as we will have to continue to accept uncontrolled immigration from the EU.

GETTY Increasing migrant numbers would not be what the public voted for with Brexit

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"Total net immigration will add 12 million people to the population in just 25 years, more than the equivalent of the entire current populations of Bulgaria and New Zealand." He added: "Theresa May needs to set out in no uncertain terms that when the UK leaves the EU it will also cease to be a member of the Single Market. "The UK will then, like the rest of the world, have access to the Single Market, and tariff free access will be hugely beneficial to the EU, which runs a massive trade surplus with the UK." The Migration Watch report cited official figures showing that annual net migration from other EU nations to the UK reached 189,000 in the 12 months to the end of last June.

GETTY As long as Britain remains in the single market, it will not fully leave the EU

Other data showed that the total number of EU migrants living in the UK has more than doubled to 3.2million since border controls were relaxed for a string of Eastern European nations. More than 1.5million citizens from former communist Eastern Bloc countries including 288,000 Romanians and Bulgarians were now living in the UK, the figures showed. Ministers could make few changes to the influx if the UK remained signed up to Brussels "free movement" rules, the Migration Watch report said. "The scope for a significant reduction in EU migration is extremely limited if the UK remains a member of the Single Market and thus remains subject to free movement of people," the report said.

GETTY The impact of a falling pound to the euro, has had little impact on migration numbers

"Looking ahead, many of the push and pull factors that have led to high levels of EU migration in the past are likely to continue." The fall in the value of the pound against the euro since the referendum vote had made little impact on the incentives for migrants to head to Britain from Eastern Europe, according to the report. Britain's monthly minimum wage of 1,343 euros (£1,120) compared with minimum monthly pay rates of 417 euros in Poland, 276 euros in Romania and 214 euros in Bulgaria. "Of course the cost of living in Eastern Europe is much lower so small savings of UK wages go a long way when sent home to family members," the report said.

GETTY The minimum living wages in the UK are attractive to many Eastern Europeans

"The UK minimum wage is also set to increase substantially by 2020, adding to the incentive." Forecasts from the Organisation for Economic Co-Operation and Development suggested wages in Eastern Europe were unlikely to rise to UK levels before 2035. High unemployment levels across many EU countries also added to the incentive for EU citizens to head to the UK, according to the report. Migration from southern European countries including Spain and Italy was continuing because of the continuing euro-zone crisis.

GETTY Brexit was a promise of change in the immigration rates and policies

"While these countries are expected to experience moderate economic growth, unemployment levels are not fully anticipated to fall to pre-recession levels for some time. "Youth unemployment in Spain is projected to remain above 40 per cent even in 2020," the report said. Other EU countries were also becoming less attractive than the UK for Eastern European migrants, the report said.

GETTY Shrinking economic opportunities in other EU states is making people come to the UK

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Romanians and Bulgarians were less attracted to Italy and Spain because of shrinking economic opportunities. And Polish workers were finding Germany less attractive because of increased labour market competition due to the arrival of hundreds of thousands of refugees and asylum seekers from the Middle East, Africa and Asia. The report concluded: "It is not possible to be both a member of the European Single Market and in control of EU immigration.

GETTY London's population is expected to grow to 10 million by 2030