The UK's population is growing significantly faster than previously estimated and will exceed 75 million by 2051, an expert has predicted.
Immigration and increased lifespan are key factors in population rise
This represents a rise of 15m by the middle of the century and is 6m higher than current projections.
But Oxford University demographics professor David Coleman says it is still probably an "underestimate".
Rising immigration, increased longevity and higher birth rates among migrant families are key factors.
The current UK population is 60m and the increase would be equivalent to two new London-sized cities.
The last official forecast in 2005 showed it rising to 69m by 2051, but Prof Coleman calculates it will reach 69m in 2031 and 75m in 2051.
He also claims the proportion of the UK population classed as non-white will grow from 9% at the last census in 2001 to 29% in 2051.
His calculations are disclosed in evidence he submitted to the House of Lords economic affairs committee and reported in The Sunday Telegraph.
The evidence was part of the committee's current inquiry into the economic impact of immigration in the UK.
It has received submissions from a wide variety of sources, including councils, retailers, banks and academics.
Prof Coleman's calculations are based on an updated model for counting migration adopted by the Office for National Statistics (ONS) and are expected to be confirmed by Government population experts this week.
But Danny Sriskandarajah, a migration expert with the Institute for Public Policy Research thinktank, said the assumption that the current rates of immigration would continue seemed "rather simplistic".
"Prof Coleman's methodology is based on what's happened in the past few years," he said.
"Immigration in recent years has been at very high levels because of the enlargement of the EU and strong labour market conditions."
He added that 20 years ago, the UK was losing more people than it was gaining and projections from then, using a similar methodology, would have shown depopulation.
Evidence submitted to the committee by the National Institute of Economic and Social Research concluded long-term immigration forecasts are associated with "substantial uncertainty".
The memo said: "With this in mind it is not a question of one forecast proving to be right and another forecast proving to be wrong.
"It is important to provide a description of the uncertainty associated with migration forecasts."
ONS estimates for the number of people migrating to the UK has increased to 190,000 a year compared with 145,000 in calculations.
This is thought mainly due to higher numbers of eastern Europeans coming to Britain after the 2004 EU expansion.
Prof Coleman used the updated ONS model for his calculations but did not factor in improvement in survival so believes his figures are "probably underestimates by one or two million".
"The absent-minded commitment into which we have drifted, to house a further 15m people, must be the biggest unintended consequence of government policy of almost any century," he told peers.
"As it is by no means unavoidable, being almost entirely dependent upon continued immigration, it might be thought worthy of discussion. In official circles, there has been none."