The population in the eastern region will top 6.4m people in the next quarter of a century - a rise of 16.8%.
The projections from the Office for National Statistics (ONS) up to 2028 have been made assuming recent population trends will continue.
But they have discounted any impact from future development policies.
The largest increases are projected to be in Cambs and Beds, though in Luton growth will be lower at less than 5%.
All counties and unitary authorities are expected to see increases of more than 10% over the next 25 years.
These new, long-term population projections for local areas are based on the 2003 mid-year estimates and replace the 1996-based population estimates previously published by the ONS.