Mass migration from the European Union (EU) will be “inevitable”, putting a huge strain on public services, if the government stays in the Single Market after Brexit, a report has warned.

The new analysis from Migration Watch says the scope for any significant reduction in immigration from the bloc would be “extremely limited”. Wage disparity, unemployment, and the future naturalisation of the huge number of third world migrants EU nations have granted asylum in recent years are among the push and pull factors the think tank has considered.

Unless freedom of movement is curbed, more than 12 million people will come to Britain in the next 25 years from the EU — more than the combined populations of Latvia, Lithuania, and Bulgaria — the report has calculated.

Migration Watch’s contribution to the debate adds to mounting pressure on Theresa May to formally rule out a ‘soft Brexit’, or membership of the single market after Britain leaves the EU.

Large wage disparities between the UK and Eastern Europe, where Bulgaria’s minimum wage is just 15 per cent of Britain’s, Romania’s a fifth, and Poland’s a third, would contribute to the influx.

With the minimum wage in the UK “set to increase substantially by 2020”, Migration Watch notes the incentive to migrate will be even higher. Additionally, it notes that the OECD doesn’t forecast the gap in regional wages even beginning to close before 2035.

Another factor considered in the analysis is the huge number of non-EU nationals granted asylum — 770,000 in 2015 alone — as these people are entitled to citizenship after qualifying periods ranging from just two years, in the case of Belgium.

The report notes: “Once granted citizenship in any EU member state they will acquire the same freedom of movement rights as other EU citizens and would be free to travel on to the UK.”

20,000 of the Somalis who were granted asylum in the Netherlands, and later naturalised as citizens in the mid to late 1990s, had already migrated to Britain by 2004. Studies show that around half the entire Dutch Somali community has moved to the UK.

Migration Watch warned the implications for the UK are “hugely significant”, and the report states: “At a time when the UK government is seeking to close the budget deficit it is hard to see where the money will come from to provide the additional schools, GP surgeries, hospitals and housing, not to speak of how the country’s road and rail networks will cope with such rapid growth.”

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.”