The UK population has topped 61 million for the first time, the Office for National Statistics said.
The population increased by a record amount to 61.4 million – with an extra 408,000 people living here in 2008.
A recession-fed baby boom has been cited for the rise, with fertility rates also increasing to their highest numbers in a generation.
The increase, which is more than twice that of 2001 when the population rose by 201,000, is the highest since modern records began in 1972.
The news has prompted a race among statisticians to confirm it is the biggest population increase in history.
Natural population changes caused by shifts in birth and deaths have, for the first time in nearly a decade, overtaken immigration as the biggest factor affecting population growth.
In the midst of the recession, the flood of Eastern European immigrants – who flocked here after 2004's EU expansion – has slowed significantly.
In the year to December last year, arrivals from the A8 countries of Eastern Europe fell by more than a quarter – 28% – from 109,000 to 79,000.
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UK National Statistics