Scotland’s population will rocket to well over six million by 2050 if growth keeps pace with the rest of the UK, a study has found.

Rapid growth could lead to huge problems in terms of infrastructure and urban overcrowding and, with almost one million pensioners, the country could struggle to pay for its elderly.

American researchers said the UK was on course to become western Europe’s most populous country, overtaking France within the next 40 years.

The UK population will hit 77 million in 2050, according to the Population Reference Bureau in Washington, with natural growth and immigration fuelling the boom.

If Scotland grows at the average UK rate, it will be home to almost 6.3 million people – up more than one-fifth, and with immigrants accounting for a far larger proportion of society.

UK Immigration Minister Damian Green warned that while incomers could be a good thing, there were also difficulties when population is allowed to change too rapidly.

He told The Herald: “Britain has always benefited from being outward-looking and having a population with diverse backgrounds.

“Problems arise in periods when the population changes too fast. That’s why the Government wants to control the level of immigration so that we can benefit from the arrival of talented people from all over the world without putting pressure on our public services.”

Currently, there are just over five million people in Scotland, of whom about 4.5% belong to ethnic minorities. This could rise to more than 15% by 2050, a separate study found.

Phil Rees, who led a separate study at Leeds University earlier this month, said it was essential that schools and maternity services were prepared for the influx. “It is a continuation of current trends in terms of the mixture of ethnicity in today’s primary classrooms, and that will move on to secondary schools and university,” he said.

Though the trend in Scotland has been population decline since the early 1970s, numbers have begun to pick up, and immigrants are increasingly seen as vital to the national economy.

Net migration to the UK is two per 1000 inhabitants, the new report says – one-quarter of the rate in Norway.

Per person, the UK takes roughly as many migrants as the Netherlands and France, but more than Germany, where more people are leaving than arriving.

By 2050, experts have suggested that factors such as climate change will add to Scotland’s appeal. A conference at the Royal Society of Edinburgh heard that areas of southern England and mainland Europe would be virtually uninhabitable as they heated up, and problems such as water shortages could become commonplace.

At the event, John Thomson, director of strategy at Scottish Natural Heritage, said: “Mass migration is a real prospect with the sort of temperature increases we’re talking about now”.

The new study said the overall trend was for populations to increase dramatically in developing countries while remaining stable or falling in richer states.

However, while the latest findings from the US predict huge population growth in the UK, a study published earlier this month forecast lower growth.

Leeds University researchers said Scotland would have 5,773,300 people in 2051, fewer than the US study suggests.

With almost one million pensioners by 2050 the country could struggle to pay for its elderly.