Six countries have been responsible for 80% of the growth in Britain’s EU-born population in recent years, research has found.

Spain, Italy and Portugal have joined Poland, Romania and Hungary as the key nations driving the increase, according to an analysis by the Migration Observatory at the University of Oxford.

They were identified as the EU member states whose UK-resident populations have grown the most between 2011 and 2015.

The researchers said that some of the factors encouraging migration to the UK are permanent, such as the attraction of the English language, while others have the potential to change over time.

For example, economic factors such as high unemployment in southern Europe and lower wages in eastern Europe are likely to be key drivers of recent migration, the study said.

Madeleine Sumption, director of the Migration Observatory, said: “There is no single factor driving high levels of EU migration in recent years.

“Some drivers are likely to remain in place for some years, such as the relatively low wages in new EU member states, particularly Romania.

“Others could potentially dissipate more quickly, like high unemployment in Spain.”

The commentary argued it is too early to tell how the introduction of the national living wage will affect migration, saying that while it may increase the financial advantage of moving to the UK from a lower-income EU country, it could also push UK employers to rely less on low-wage workers, including those from the EU.

Sumption added: “Despite recent debates about the role of UK policies like welfare benefits or the minimum wage in driving migration, migration may respond more to factors that governments don’t directly control, like demographics and economic growth in other EU countries.”

Migration is a central theme in the debate over Britain’s membership of the 28-country bloc before the referendum in June.

The latest official data shows estimated net migration of EU citizens to the UK was 172,000 in the year ending September.

Earlier this month it emerged that new figures on the numbers of active national insurance numbers (Ninos) given to EU nationals are set to be published.

The move followed controversy over a discrepancy between estimates of long-term migration to Britain from the EU and the numbers of Ninos handed out over the same period.

The research comes as another report predicted that immigration is unlikely to fall substantially if Britain left the EU. The study by the free-market thinktank Open Europe found that other large developed countries with low unemployment outside the EU also had high migration.

It points out that Canada, Norway and Switzerland, all countries sometimes highlighted by Brexit campaigners as representing a model for a better relationship with the EU, have all experienced higher levels of net immigration.