The previous record of 10.03C was set in 2022.
"2025 now joins 2022 and 2023 in the top three warmest years since 1884," the Met Office said in a statement, noting the United Kingdom's mean temperature through last year was 10.09C.
"This is an increasingly clear demonstration of the impacts of climate change on UK temperatures," it added.
"It is also only the second year in this series where the UK's annual mean temperature has exceeded 10.0C."
It means four of the UK's last five years now appear in the top five warmest years since 1884, and all of the top 10 hottest years will now have occurred in the last two decades.
The Met Office had already announced last month that 2025 was the country's sunniest year since that record series began in 1910.
The United Kingdom, which comprises England, Scotland, Wales and Northern Ireland, saw 1648.5 hours of sunshine, 61.4 hours more than the previous record set in 2003.
- 'Exceptional' sunshine -
An "exceptional" amount of sunshine during the spring followed by long spells of clear skies during the summer helped set the record, the Met Office has noted.
Mark McCarthy, the Met Office's head of climate attribution, said the "very warm" year was "in line with expected consequences of human-induced climate change".
"Although it doesn't mean every year will be the warmest on record, it is clear from our weather observations and climate models that human-induced global warming is impacting the UK's climate," he added.
Bob Ward of the Grantham Research Institute on Climate Change and the Environment at the London School of Economics, said the record was "undoubtedly due to climate change".
"We can only stop the growing impacts of climate change by cutting our greenhouse gas emissions to net zero, and the longer we take to reach it the more death, damage and suffering there will be," he said.
The country experienced persistent spells of dry and sunny weather in 2025, with every month except January and September warmer than average.
- Wildfires -
Spring and then a summer featuring four heatwaves were Britain's warmest on record, while spring was the driest in more than a century.
Droughts were declared in several regions, reservoir sites fell below 50 percent of their usual capacity and a handful of water companies issued bans on using hosepipes.
Fire brigades in England in 2025 also tackled the highest number of spring wildfires since comparable records began in 2011, according to figures obtained by the PA news agency from 35 of the 43 fire services in England.
At least 12,454 grassland, woodland or crop fires were recorded by fire services in the three months from March to May 2025.
The number was more than four times the 2,621 incidents logged in the same period in 2024.
Met Office scientist Emily Carlisle said: "Meteorologically, the warmth has been driven largely by persistent high-pressure systems bringing prolonged dry, sunny conditions, alongside above-average sea temperatures around the UK.
"These factors have combined to keep temperatures consistently higher than normal for much of the year."
Last year Britain produced a record-high amount of electricity from renewable energy.
Wind, solar and biomass power combined to supply 47 percent of the country's electricity in 2025, climate and energy website Carbon Brief said in a study Friday.
The country has set a target to reduce its greenhouse gas emissions by at least 81 percent by 2035 compared to 1990 levels and is aiming to be carbon neutral by the middle of the century.
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