A new report, published by an influential think tank, claims that the impact of immigration on the British population has been underestimated by more than 1.3 million.
MigrationWatch UK has argued that official statistics fail to take into account the large number of babies being born to mums from overseas.
Their study found that 84 per cent of population growth between 2001 and 2012 was due to migration, once births by foreign mothers were factored in.
The Office for National Statistics had previously claimed that, based on its own analysis, migration accounted for just 57 per cent of population growth over that 11 year period.
Sir Andrew Green, the MigrationWatch chairman, said: “This is about population growth, not about citizenship.
“Those born in the UK to settled immigrant parents are British citizens, irrespective of their parents’ country of birth. That said, it is now undeniable that the massive scale of net migration has been the main cause of our population growth and that, in the future, our population growth is likely to be almost entirely due to migration.
“The conventional official statistics [published by the ONS] do not make this clear.”
The think tank added that unless net migration is significantly reduced, the UK will have to build the equivalent of ten cities the size of Birmingham over the next 25 years.
Some, however, have been sceptical about MigrationWatch’s calculations – a previous report by the organisation was described as “a stark misapprehension” by critics.
An ONS spokesman said: “In accordance with United Nations international standards, ONS does not define children who are born in the UK as immigrants.
“MigrationWatch has used a different definition which produces a higher total.”