Rain and thunderstorms are set to make way for sunshine and warmth in time for Glastonbury this weekend, according to the Met Office.
The June washout will continue on Monday and Tuesday, with severe Met Office weather warnings for rain and thunderstorms covering large parts of the UK.
However, hot air moving in from continental Europe will bring with it drier, sunnier and warmer conditions, and by Saturday parts of London could get as hot as 35C.
The June record is 35.6C, which was set in Southampton in 1978.
As the week progresses the heat is forecast to build, with southern areas getting the hottest weather.
Peak daily temperatures across much of England and Wales are predicted to be well above 20C all week.
Scotland and Northern Ireland will be slightly cooler, although the mercury is expected to rise above 20C on Saturday.
Met Office spokeswoman Nicola Maxey said: "The continent is seeing some very high temperatures, with record-breaking temperatures expected across France, Spain and Belgium.
"We are not seeing temperatures as hot as Europe, but it will be warm for the UK."
She added: "By Saturday we could be looking at 30C in the south, with London looking at 30C but with isolated spots of 33C, 34C or 35C, maybe."
Last year's record for June, a month in which the UK was hit by a series of wildfires, was 33C recorded at Porthmadog in Gwynedd.
Glastonbury festival-goers will be treated to consistently warm and dry weather, with temperatures expected to largely remain in the mid to high-20s throughout the weekend.
On Friday it could get as hot as 31C at the music festival.
Both the UV index and the pollen count are forecast to remain high this week.
Ms Maxey said: "People who suffer from hay fever might feel quite uncomfortable."
The highest temperature of the year so far was 28.8C, recorded in Norfolk on June 2.