Downing Street has said forecasts indicating Britain’s population will pass the 70 million mark before the end of the next decade prove the need to bring net migration down.

Theresa May’s spokeswoman said the data suggesting more than three-quarters of the increase would be linked to newcomers from overseas justifies work to reduce migration to “sustainable” levels.

The Government has promised to reduce net migration to the tens of thousands, with more details of the post-Brexit immigration regime expected soon.

Data published by the Office for National Statistics predicts that between 2016 to 2041, the population will grow by 7.3 million, equivalent to the current population of Scotland and Northern Ireland combined.

The Prime Minister’s spokeswoman said: “The stats today are projections based on current trends. I would say that they confirm that we are right to continue our work to bring net migration down to more sustainable levels.”

Demographers project that the population will rise by 3.6 million, or 5.5 per cent, over the next 10 years, rising from an estimated 65.6 million last year to 69.2 million in mid-2026.

The ONS said the population was projected to pass 70 million by mid-2029, reaching 72.9 million in 2041.

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This is two years later than the ONS projection in 2014, which stated the population would pass the 70 million barrier in mid-2027.

Reasons behind the revised rate are that since 2014 assumed net migration has been lowered, that the predicted number of children per woman has been reduced and that the projected rate of increase in life expectancy has been cut.

Andrew Nash, of the ONS Population Projections Unit, said: “These projections suggest slower growth than the previous projections.

“This is because of lower assumptions about future levels of fertility and international migration, and an assumption of a slower rate of increase in life expectancy.”

The report also reveals that the rise in the number of inhabitants in the UK is tracking at well above the EU average, and at the fastest rate among the four largest nations in the bloc.

Projected UK growth of 16 per cent between 2015 and 2040 compares with 10 per cent for France and 4 per cent for Germany, while Italy’s population is projected to see a slight decline.

The study gives a fresh indication of the ageing nature of the population. In mid-2016, there were 1.6 million people aged 85 and over – by mid-2041 this is projected to double to 3.2 million.

The ONS emphasised that the projections do not attempt to predict the impact of the UK leaving the EU, which is already starting to have some effect on migration.