Highs of 29.5C were seen in London and the South East while Scotland recorded its hottest day of the year so far.
The temperature reached 28.5C in St James’ Park, London, on Friday, which made the capital hotter than Los Angeles and Santorini.
Firefighters are warning that there is an increased risk of fires due to the heatwave.
If the prediction is accurate, Britain will be hotter than Portugal, Jamaica, Costa Rica, the Canary Islands and Cyprus.
Met Office says a few areas could see ‘in excess of 30mm of rain in a couple of hours, possibly accompanied by lightning and hail’.
Hay fever sufferers are set to experience more extreme symptoms over the next few days.
Speeds of up to 45mph could hit the far north-west of the UK on Friday.
The Met Office issued a yellow rain warning in eastern parts of England lasting until 4am on Monday.
The Met Office is considering a thunderstorm weather warning for Saturday night as crowds gather for the BBC’s Platinum Party at the Palace.
Britain basked in sunshine on Monday, but a high pollen count could cause discomfort for those with tree allergy hayfever.
With temperatures climbing into the mid-20s, people have been advised to stay out of the sun and wear sun cream.
It comes after an April that has seen average maximum temperatures generally above normal for the month.
Temperatures may reach 23C (73.4F) in the UK while the European nations could see highs of 19C (66F) and 21C (70F).
Temperatures are expected to peak at 22C in some parts of the UK
Forecasters said the ‘peaks and troughs’ of spring will hit most of the country over the next week, with sunny spells and windy intervals.
Human beings have become very good at predicting the weather - but despite various attempts, we still can’t control it.
The Met Office is expecting above average temperatures for the week ahead, after reaching 20C in some areas on Saturday.
Chilly and wet conditions are set to give way to dry and bright weather and a hint of spring warmth in the coming days.
The Environment Agency said that severe flooding is expected to continue.
The third named storm in a week has brought travel disruption, with a yellow wind warning in place for much of the UK until 1pm.