A baby boom, longer life expectancy and higher-than-expected immigration over the past decade have led to the single-largest population surge since records began.
England and Wales are home to 53.1 million people - up 3.7 million on 10 years ago - according to the results of the 2011 Census released yesterday. Just over half the increase was down to new settlers, according to the Office for National Statistics. The figures also show an ageing population, with one in six Britons now over 65.
The overall population rise is sharper than expected, and means there are 476,000 more people living in England and Wales than previously estimated. This is due to higher-than-expected immigration rates, combined with under-counting at the last Census in 2001, the ONS said.
But the country's population boom is by no means evenly spread across the two countries, with over half of the population growth concentrated in London, the southeast and the east. In fact, with some notable exceptions such as Greater Manchester, many towns and cities in the north actually saw populations stagnate or decrease.
The Census shows that on average 371 people live in every square kilometre of the country, although this figure is skewed by London, where 5200 residents pack into each kilometre.
Matt Cavanagh, visiting fellow at the Institute of Public Policy, said: "It is really only London and the southeast which is crowded ... Today's figures show that some local authorities, particularly in the north, are struggling with the opposite problem."
Simon Ross, chief executive of Population Matters, said: "England faces unsustainable pressure on housing, roads and public transport and green spaces ... We should give an example to other countries by seeking to limit our numbers."
The biggest population swells were among the very old and very young. There were 3.5 million pre-school children in 2011 - 406,000 more than 10 years earlier. The birth rate is higher among new migrants because they are usually of working age, but is also rising among British-born women from different ethnic backgrounds.
For the first time, one in six Britons is now over 65, while the median age is 39, up from 25 a century ago. The biggest rise was among the over 90s reflecting the increased longevity public policy makers fear due to demand for healthcare.