Britain has seen its warmest March day for five years, with the temperature in Gravesend, Kent, reaching 22.1C.
Thursday is also the warmest day so far this year, beating Ibiza and Barcelona (both 19C) and Nice in France (18C) and Istanbul in Turkey (17C).
London enjoyed some of the best weather, with 21C recorded at Heathrow, High Beach, Epping Forest and St James's Park during the afternoon.
Met Office spokeswoman Nicola Maxey said: "We've got some warm air coming in from the continent which is clipping the southeast corner of the UK, so we're seeing some temperatures certainly in the high teens away from that corner, but it's only London and parts of East Anglia reaching these higher temperatures over 20C (68F)."
Previously, the warmest day this month had been the 19.9C reported in Aviemore in the Scottish Highlands on Sunday.
The warmest March day ever was recorded in 1968, with a high of 25.6C recorded in Mepal, Cambridgeshire.
Sky News Weather Producer Kirsty McCabe said Thursday evening would see spells of rain in parts of Northern Ireland, Ireland, southern Scotland, western Wales and northwest England.
This rain will continue to push northeast during Friday, while the rest of the country remains mostly dry and sunny but with lower temperatures than those seen on Thursday.
The end of March usually sees temperatures of around 13C, while April, known for showers, could start with heavy rain, thunder and hail over much of the country on Saturday.