Temperatures in Wales and the west country, including festival-goers at Glastonbury, could see temperatures of 30 or 31C on Friday.
In west Scotland it could reach 27 or 28C and in Northern Ireland 25C, while in Aberdeen it will be a cooler 17C and in London and down the east coast 24C.
After a humid Friday night, Mr Dewhurst said Saturday would be a case of “role reversal”.
The highest temperatures will be in the east, with a maximum of 34C possible in an area covering London and up towards Lincolnshire.
“We will see the hottest day of the year so far,” said Mr Dewhurst.
He said the sharp rise in UK temperatures was not a heatwave, which are typically considered to last three days or more.
“It’s more just a hot spell rather than a heat wave,” he said.