Migration experts have challenged new official projections that Britain's population will hit 70 million by 2029, largely as a result of a future influx of people and births to foreign nationals in the UK.

Tim Finch, head of the Institute for Public Policy Research's migration programme, said the Office for National Statistics projections published today assumed migration patterns over the next 10 years would mirror those of the last 10.

The ONS figures are based on the average pattern of migration into Britain over three years up to the middle of 2008, before the economic recession began to bite. "Early indications suggest that the peak of net migration, mainly from eastern Europe, has passed," said Finch. "The next few sets of migration figures will suggest that inward migration is steadying and emigration is increasing. If that trend continues then some of the assumptions that net migration will go on and on that lie behind the projection of 70 million by 2029 may be wrong."

The ONS projections show that the current UK population of 61.4 million would rise to 71.6 million by 2033, passing the 70 million mark by 2029. Should that happen it would be the fastest rate of population growth seen since the postwar baby boom, with the ONS saying it would mean adding a city the size of Bristol to the population each year.

But claims that a new "immigrant baby boom" are fuelling the rise appear wide of the mark. One in four babies born last year had non-British-born mothers. This partly reflects the younger age profile of recent migrants and the greater prevalence of women of childbearing age, and a higher fertility rate than among British-born women.

Statisticians say 55% of the 10.2 million projected rise over the next 25 years will come from a natural increase in births over deaths, and 45% will be due to more people coming to Britain than leaving.

The home secretary, Alan Johnson, said earlier this year that he did not "lie awake at night" worrying about the population hitting 70 million.

Guy Goodwin, of the ONS, stressed that the figures were not forecasts and did not "take account of new or future policy initiatives". "Really, they're just a benchmark that policy-makers and politicians can look at and say, 'This is where we are heading if things continue very much as they are.'"

The projections are actually lower than the set published last year and are based on the immigration picture in 2006, 2007 and provisional figures up until the middle of 2008. The ONS has revised net migration downwards by 10,000 to 180,000 a year and projected that the 70 million mark will be passed a year later, in 2029, compared with last year's projections.

Phil Woolas, the immigration minister, said the ONS projections showed that population growth was starting to slow and reforms that the Home Office had made to the immigration system over the past two years were working.

"Last year saw a 44% fall in net migration and we expect that fall to be sustained and reflected in future projections."

The ONS also suggests the oldest age group is likely to grow the most quickly, with the number of Britons over 85 due to more than double over the next 25 years, from 1.3 million in 2008 to 3.3 million by 2033. The number of centenarians is due to rise from 11,000 to 80,000 by 2033.

Even taking account of increases in the state pension age, those qualifying for a state pension are expected to rise from 11.6 million in 2008 to 15.6 million by 2033.

New life expectancy figures confirm the north-south divide in Britain. Life expectancy is highest for men at 79.2 years in south-east England and for women at 83.1 in south-west England. It is lowest in Scotland, at 75 for men and 79.9 for women.

Life expectancy at birth has improved across the UK since the early 1990s, but while London saw an increase of 4.9 years for men, life expectancy for Scottish men rose only by 3.5 years.

A boy born this year in Kensington and Chelsea can expect to live for 84.3 years compared with only 70.7 years for a boy born in central Glasgow.