New figures from the Office for National Statistics (ONS) show a boy born between 2018 and 2020 is expected to live until he is 79 years old.
This is a slight fall from the life expectancy for a boy born 2015-17, who was expected to live until he was 79.2 years old.
Life expectancy for women, however, remains unchanged, with girls born in the past three years expected to live 82.9 years, the same as in the previous period.
The figures reflect the impact of the Covid which led to a greater number of deaths than usual last year, the ONS said.
Life expectancy had been steadily rising for both men and women since the ONS first began collecting comparable data in 1980-82.
Then, a newborn boy was expected to live for 70.8 years, while a woman would live on average for 76.8 years.
Pamela Cobb, of the ONS’s centre for ageing and demography, said: "Life expectancy has increased in the UK over the last 40 years, albeit at a slower pace in the last decade.
"However, the coronavirus pandemic led to a greater number of deaths than normal in 2020.
"Consequently, in the latest estimates, we see virtually no improvement in life expectancy for women, while for men life expectancy has fallen back to levels reported for 2012 to 2014.
"This is the first time we have seen a decline when comparing non-overlapping time periods since the series began in the early 1980s."
However, statisticians have stressed the fall in life expectancy does not mean a baby boy born last year is guaranteed to have a shorter life than his brother born a few years earlier.
The slight drop in the figures is a reflection of the increase in overall mortality caused by the pandemic, and this rise in death rates would need to be sustained into the future to actually drag down everyone’s life expectancy eighty years from now.
“These estimates rely on the assumption that current levels of mortality, which are unusually high, will continue for the rest of someone’s life," Ms Cobb added.
"Once the coronavirus pandemic has ended and its consequences for future mortality are known, it is possible that life expectancy will return to an improving trend in the future."
There are also slight variations in life expectancy between the four nations of the UK too.
In Scotland, male life expectancy also fell from 77.0 to 76.8, and female life expectancy also saw a slight decline from 81.1 to 81.0.
However, in Northern Ireland, life expectancy for both men and women rose, from 78.4 to 78.7 for males and 82.3 to 82.4 for females.
In Wales, life expectancy dropped slightly for women (82.3 to 82.1) but was unchanged for men (78.3 years).
Previous data from the ONS shows that about 80,000 more people died in England and Wales in 2020 than average for the previous five years.
This statistic, which is notably higher than official Covid deaths, underlines the impact of the pandemic across society and the healthcare system beyond those who died directly from the virus.
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