The UK population could almost double over the coming 75 years, according to official government projections.
The working population is projected to rise over the coming decades
The previously unpublished figures suggest the British population could hit almost 110m in 2081, if immigration fertility and longevity rates are high.
The figures are higher than those released just a month ago by the Office for National Statistics.
In October, the ONS projected the population could go from around 60m today to as high as 77m in 2051.
The new projections are supplementary figures to help policy makers plan for spending on pensions and the welfare state.
The ONS comes up with different models of how the population could change, based on variations in migration, birth rate and life expectancy.
According to the ONS, if all of these factors were on the high side over the coming decades, the population across the UK would hit 91,053,000 by the middle of the century - and 108,723,000 by 2081.
If migration alone proved to be high, then the population would top 75m by 2031 and rise to 92m by 2081.
At the other end of the scale, the ONS suggests that low fertility, migration and life expectancy would see population rise by just four million over the next 75 years.
The principal projection - the scenario it thinks to be most likely - puts the population at 71m in 2031 and 85m in 2081.
Experts draw up such wide variations because they say it is extremely difficult to accurately predict how the population will change beyond the next few years.
But critics of the government's migration policies have previously questioned how these calculations are presented, suggesting they underplay the impact of current rates of migration.
Background data relied upon by government suggests the birth rate could be pushed up by relatively young migrant workers.
This could mean 69% of Britain's predicted population growth could be associated directly, or indirectly, to immigration - although the figures are tentative.
The new figures project that the net population growth from immigration to be probably just short of 200,000 a year until 2021 - but adds it could peak in the near future at 300,000 a year.
Separate statistics released earlier in November show that the UK is experiencing record levels of migration both into and out of the country.
The figures from the Office for National Statistics suggest 591,000 people migrated to the UK in 2006 while some 400,000 people moved overseas.
Sir Andrew Green of Migrationwatch UK said that the projections were a warning that should be heeded.
"These projections are a sharp reminder of what could happen to our population, indeed our country, if the government fails to take firm and effective action," said Sir Andrew.
"They have consistently underestimated the scale of immigration. They have had to raise their assumptions about future immigration six times since 1997. We simply cannot afford any more misjudgements of this kind."