There has been a Six-fold increase in UK-born residents holding both British and Irish passports since 2011.
The figures from the Office of National Statistics also show the median age of UK/Irish dual citizens was 47.
This was much older than those with an Irish-only passport, which was 33.
A rise in UK/Irish dual citizens across the decade was most apparent in those aged 50 to 70-years-old, the ONS said, adding that this suggested many only took up their dual nationality more recently despite moving to England and Wales years ago.
The top five UK/EU passports for UK-born dual citizens were primarily held by children or young adults, with Ireland having the only older population, the ONS said.
The number of people with multiple passports in England and Wales has doubled in a decade.
There was a five-fold increase in people born in the UK holding both British and EU passports, the analysis of Census 2021 figures showed.
While the change has been partly driven by migration over the decade, with more people moving to the UK from the EU, it is also thought more people have taken up additional passports after Brexit, the Office for National Statistics (ONS) said.
In 2021, a total of 1.26 million usual residents of England and Wales held multiple passports (2.1%), up from 612,000 (1.1%) in 2011.
The majority – 84.4% or 50.3 million – of usual residents across the two nations held only one passport in 2021.
While the number of UK/EU passports held increased by five times over the 10-year period for UK-born residents, there had been a three-fold rise for non-UK-born residents since 2011.
Jay Lindop, from the ONS, said: “The number of people with dual citizenship has doubled since 2011, with more than 1.2 million usual residents in England and Wales now holding multiple passports.
“This change has been partly driven by migration over the decade, with an increase in people moving here from the EU. As people who have settled in England and Wales go on to have children, we can see an increase in dual citizenship among the younger ages.
“The rise in dual citizens may also suggest greater uptake of additional passports following the end of free movement when the UK left the European Union.”
There were age differences among dual citizens, with the rise in UK-born British and European dual citizens driven by younger age groups, but British/Irish passport holders being older, the ONS said.
More than half (58.7% ) of dual citizens born in the UK and holding both UK and EU passports were aged under 16, while just over a third (36.7%) of UK and non-EU dual citizens were under 16.
UK/EU dual citizens had a median age of 12, while for EU-only passport holders it was eight-years-old on average.
Non-EU dual citizens had a median age of 22.
For UK/French and UK/German passport holders the median age was 15, for UK/Polish it was seven and for UK/Italian it was 20.
The five most common non-EU passports for UK-born dual citizens showed a wider variety of average ages, with UK-Australian having a median age of 39, UK-US being 21, UK-Nigerian was 19, UK-Canadian was 36, and UK-New Zealand was 28.
The ONS said the young ages of most UK/other dual citizens suggested they were the children of first-generation migrants.
The statistics body said the reasons for holding multiple passports were likely to differ between those who held a UK passport first and those who held a non-UK passport first, noting that the census did not collect data on when passports were acquired.
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