In addition to high daytime temperatures, it will remain uncomfortably warm overnight, only dipping to 24C.
Previously the highest temperature of the year had been set on Thursday, with 32.6C recorded in Wisley, Surrey.
The UK Health Security Agency has issued an amber heat health alert, meaning weather impacts are likely to be felt across the health service, with those aged above 65 or those with pre-existing respiratory or cardiovascular disease at greater risk.
Meanwhile Mayor of London Sadiq Khan has issued an air pollution alert, the first since June, and urged Londoners to stop their engines idling and refrain from burning wood or garden waste.
“On Saturday, alongside the current high temperatures, we will also experience high levels of air pollution,” Mr Khan said.
“Pollution and heat can be a dangerous combination, which is why I’m urging Londoners to look after themselves, stop their engines idling and refrain from burning wood or garden waste, all of which contribute to high levels of pollution.
“This is particularly important in order to protect those who are most vulnerable and help us to build a safer and greener London for all.”
The Met Office is forecasting temperatures to reach 31C in London on Sunday before dropping to 25C on Monday, marking the end of a record sizzling week.
Heathrow has reached 32.7 °C 🌡️
Provisionally making today the warmest day of the year 🌡️
It is also the 6th day in a row the UK has recorded a temperature over 30 °C 🌡️ pic.twitter.com/UlrdrRn9ka
— Met Office (@metoffice) September 9, 2023
Across the UK there is potential for thunderstorms and downpours.
Met Office Chief Meteorologist Paul Gundersen said: “Although much of the UK will see high temperatures and sunny skies continue on Saturday, in what has a possibility of being the hottest day of the year so far, there’s also the potential for some thunderstorms, which has resulted in a Yellow Warning being issued for much of central England and parts of east Wales.
“Temperatures will begin to trend downwards from Saturday in the far northwest of Scotland, with a cold front gradually moving south through the weekend, bringing with it the risk of some heavy and thundery downpours on Sunday as well.
“However, the southeast will hold on to the high temperatures the longest and could still reach 32C on Sunday.”