A baby girl born in 2019 is now expected to celebrate three fewer birthdays on average, than under previous calculations.
Official 2014 data thought that girl would make it to 93.6. Now the figure is 90.4.
The report also slashed the likelihood of children reaching 100.
Although life expectancies have been and are still improving, experts say previous estimates were too high.
The improvement is much smaller than previously thought, as part of a widely acknowledged slowdown in life expectancy since 2011.
In 2018, life expectancy growth stalled for the first time in more than 30 years.
This has led statisticians to re-evaluate their assumptions about future improvements in life expectancy, resulting in the figures released today.
The ONS report calculates the impact of this less-rosy picture on children's prospects of a long life.
So a boy born in 2019 is now expected to live for 87.8 years.
But the 2016 data thought he would reach 89.7 and the 2014 data said 91.1.
And looking to the future, to children born in 2043, there is a dramatic drop in the chances of reaching 100.
The latest report says:
But the projections two years ago thought:
The ONS said: "There has been considerable public debate about the causes of the slowdown in life expectancy improvements.
"Researchers have suggested a range of possible explanations for the slowdown... several factors are at play, none of which can be singled out as being the most important with any certainty."
Many reports, including by Public Health England and the Health Foundation think tank, have attempted to get to the bottom of the issue.
A lack of a recent blockbuster moment in medicine could be an issue.
Life expectancy in the 20th Century improved with the creation of the NHS, falls in smoking, childhood immunisation (the last case of polio in the UK was in 1984) and medical advances particularly for the big killers - heart disease, stroke and cancer.
But now dementia is listed as the leading cause of death and it is incurable.
Public Health England says a more elderly population - with dementia and other long-term health problems - may also be more vulnerable to diseases like flu.
But there are issues affecting life expectancy well before old age. Deaths from drug misuse, with Scotland having the highest drug death rate in the EU, are also quoted.
One of the most politically charged questions has been around austerity - the programme of government cuts that coincides with the slowdown in life expectancy.
The evidence either way is hotly contested.
But Public Health England's report says the poorest people have felt the impact on life expectancy the hardest and that "could indicate a role for government spending".
Stalling life expectancy in the UK has attracted plenty of attention from academics, but they offer no definitive answers on the causes.
When you are talking about shifts in predictions of lifespans, it needs more than a few years of data.
But there is concern about why it's a different story to that in most other developed economies.
An analysis by the ONS last year concluded that the slowdown in life expectancy growth in the UK since 2011 was one of the largest of the countries analysed.
That's led to speculation on UK specific factors.
Cuts in government spending in the policy period dubbed by some as "austerity" might, according to some commentators, have been a factor.
It's worth noting, though, that cuts in social care in England were not replicated to the same extent in other parts of the UK.
The decline in living standards and the reduced ability of some households to pay for heating and food in the decade since the financial crisis in 2008 have also been mentioned.
The gap between life expectancy in the richest and poorest neighbourhoods in England has increased according to research last year.
Ministers have strongly denied that Government policy played a part.
The debate will continue though it may take a while before firm trends and causes can be identified.
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