Parts of the UK will be as hot as southern Spain when the mercury peaks at over 30C (88F) possibly as early as tomorrow.
The heatwave is expected to last until Saturday and forecasters expect temperatures to hit record levels again before then.
The highest temperature recorded in the UK so far this year had been 29.1C (84F) at St James Park, central London, on April 19, and it was at the same spot where the mercury hit 29.4C this afternoon.
May 29 saw Scotland and Northern Ireland enjoy their hottest days of 2018, recording 27.5C (81F) and 25.3C (77F) respectively, while Wales’ high of 26.8C was on May 27.
Although this week will be one of the hottest of the year, temperatures will not top those seen last June, according to the Met Office.
“Last June we had five days in a row above 30C which the hottest spell since 1976,” spokesperson Grahame Madge said. “We have got a good run this week and we’ve got a good chance of hitting 30C.”
He added: “This might not be the week to lay the patio.
“Although some will be relishing the hot weather, it will be uncomfortable for others. People with breathing difficulties should stay cool, and everyone needs to stay hydrated.”
The TUC has called on bosses to make sure staff working outdoors are protected from the sun.
The trade union’s general secretary Frances O’Grady said: “We all love to see the sunshine, but working outdoors in sweltering conditions can be unbearable and dangerous.
“Bosses must ensure their staff are protected with regular breaks, lots of fluids, plenty of sunscreen and the right protective clothing.”
Southern Rail said its services have been affected by the hot weather.
“With exceptionally warm weather this week and rail temperatures expected to reach 47C in many locations, Network Rail has advised that emergency speed restrictions are required,” it said.
“The primary location of these speed restrictions is between London Waterloo and New Malden on lines heading away from London Waterloo, resulting in service alterations and cancellations.”