A high of 25C was recorded in England, with top temperatures in Scotland, Wales and Northern Ireland reaching 23.6C, 23.5C and 21.4C respectively.
It comes after Wales, Scotland and Northern Ireland each enjoyed their warmest Easter Sunday on record, the Met Office said.
Sunshine and "very warm" weather is forecast for Tuesday, although there is a chance of showers in the west and southwest.
But it is likely to get cooler by midweek with scattered thundery showers moving northwards and the chance of gales in the west.
Next weekend could be "very unsettled, with often heavy rain and the risk of gales, especially in the west and southwest," the Met Office said.
Thunder and hail are likely and the balmy temperatures felt over Easter will be a thing of past.
The record-setting weather comes amid week-long protests in London by activists from Extinction Rebellion, who are demanding the UK government do more to tackle climate change.
But the Met Office warned the unseasonably warm weather should not be ascribed to manmade global warming.
"It’s worth bearing in mind that Easter is really late this year, and last year we had hotter temperatures around 19 April, to around 28C," Alex Burkill told The Independent.
He added: "Whilst it is record-breaking for Easter, it’s not even record-breaking for the date.”
It comes two months after the UK saw its hottest-ever February temperature, when Kew Gardens in southwest London reached 21.2C.
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