Many were left baffled after receiving the amber snow alert for London, south-east England, and south Wales - despite temperatures reaching 21C (70F).
"Warmest day of the year and Met Office email me an amber snow warning. Are they having a giraffe or what?!", one Twitter user, Howard Bannister, said.
The Met Office blamed a "technical error".
The snow alert for Thursday evening was a test warning that accidentally went live on the Met Office website and mobile phone application, it said.
It was also emailed to subscribers, with many taking to social media in confusion.
"As if my dad just got an email from the Met office with an amber warning for snow in the south west. Mate, have you looked out the window?" wrote Twitter user Briony.
Essex Weather Centre tweeted: "Looks like the sun has got to someone at the Met Office! Definitely no snow expected this evening or overnight!"
The forecaster later confirmed that Thursday had been the warmest of the year so far, with temperatures expected to continue to rise over the weekend.
Sunday is expected to be the hottest day, with the temperatures expected to reach 26C (79F) in London and 21C (70F) in Edinburgh - hotter than parts of Spain and Los Angeles.
The average UK temperature for May is 14C (57).
However, some showery rain is expected on Friday night in Northern Ireland and Scotland, and thundery downpours are predicted over parts of the west Midlands and Wales, the Met Office said.
There are also predictions from the Department for Environment, Food and Rural Affairs (Defra) of increased levels of pollution on Sunday, with high levels expected in Northern Ireland and Scotland.
The Met Office "mistakenly" issued a warning of snow on what was the UK's hottest day of the year so far.