Runaway migration will drive the UK population above 70million in 15 years – three years earlier than previously predicted.
In 2009 official estimates predicted it would take 20 years to reach this landmark level, a figure the Prime Minister has said the nation must not reach.
But the latest calculation suggests it could hit 70.4million in 2026, placing huge pressure on public services and housing.
The projection was compiled by the House of Commons library after questions by Tory MP James Clappison.
It assumes net migration - the difference between numbers arriving and leaving - remained around its current record level of 240,000 a year.
Immigration minister: Damian Green said: 'We are fixing a broken system'
Ministers pledged to cut it to the 'tens of thousands' by 2015.
Two-thirds of the population rise is due to immigration, including an 'immigrant baby boom' caused by higher-than-average birth rates among migrant mothers.
Commentators said the analysis shows the 'absolute necessity' of cutting the number of migrants coming here.
It shows the population, now thought to be around 62.8million, will rise to 65million in 2016, 67million in 2020 and 69million in 2024.
The vast majority of the population increase would be in England.
Today's figures show that even if net migration fell to an average of 180,000 a year, the population will hit 70million in 2029.
Immigration Minister Damian Green insisted net migration would fall to below 100,000 a year.
'We are fixing the broken immigration system.
'Our reforms will bring net migration down to the tens of thousands.'