It is also the first time the country has seen a high over 20C during winter, according to the weather service.
The previous all-time record for February – 19.7C – was logged in Greenwich in 1998.
The balmy weather has been enjoyed across Britain, with places as far north as Carlisle seeing the mercury soar into the high teens today.
Grahame Madge, a Met Office spokesman, told The Independent earlier on Monday that the rising temperatures were putting the UK-wide record within touching distance.
Mr Madge said the mild conditions had been partly caused by high pressure drawing up warm air from as far south as the Canary Islands and even the tropics.
But he cautioned against assumptions the unusual temperatures were down to climate change, as short-term, seasonable weather patterns were difficult to ascribe to global warming.
“One effect we expect to see is temperatures rising. Whether climate change has given these conditions a nudge, it is the sort of pattern we would expect to see.
“What meteorologists will be looking for now is whether the weather pattern itself is a feature of climate change or whether it is just a little bit warmed because of the background warming.”
The BBC weather forecaster Tomasz Schafernaker called Monday's temperatures “utterly astonishing”. The broadcaster’s weather service said it amounted to a winter “heatwave”.
Today’s record high also marks the earliest Britain has ever reached 20C in the calendar year: the previous record was set on 2 March 1977.
The unseasonably warm weather is forecast to continue throughout Monday and Tuesday, before giving way to cooler conditions on Wednesday.
A new low pressure system building over the Atlantic is expected to push the warmer air away from Britain.
By the weekend the Atlantic systems look likely to bring spells of wet and windy weather across all parts of the country, according to the Met Office.
Forecasters warned snow could even make a return over higher ground in the north.
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