Sweltering conditions will peak on Monday with temperatures due to hit 35C, as hot weather persists across England.
Sunday reached 29C in parts of south-east England, but Monday will be even hotter, with highs of 34C in central and eastern England, the Met Office said.
The south-east could then hit 35C on Tuesday, forecaster Matthew Lehnert said, which would be hotter than Barbados.
Wimbledon is also set for its hottest opening day on record on Monday and could even see the highest temperature ever recorded during the tournament.
The warmest opening day on record was on 25 June 2001, when temperatures hit 29.3C.
Tropical nights are also in store for many, with temperatures overnight not dropping below 20C, Mr Lehnert said.
Fresher air is then expected to move in from the west in the middle of the week, bringing an end to the heatwave.
The hottest day of 2025 so far was recorded on 21 June in Charlwood, Surrey, when temperatures hit 33.2C.
It comes as an amber heat health alert, covering London, the East Midlands, South East, South West and East of England, is in place until 6pm on Tuesday.
The UK Health Security Agency (UKHSA) also issued a yellow alert for Yorkshire and Humber and the West Midlands for the same time period, with the agency warning of significant impacts across health and social care services.
An official heatwave is recorded when areas reach a certain temperature for three consecutive days, with thresholds varying from 25C to 28C in different parts of the UK.
London Fire Brigade assistant commissioner Thomas Goodall warned of wildfires in the capital during the sweltering heatwave.
So far this year, firefighters have responded to 14 wildfires in London, and there have been “countless call outs” to smaller fires involving grass, trees and in other outdoor spaces.
He urged the public to “act responsibly” to prevent fires.
There will be a “marked difference” in northwestern parts of the UK, however, with cloud and heavy rain in parts of Northern Ireland and Scotland, where temperatures will stay in the mid to high teens, Mr Lehnert said.
Meanwhile, continental Europe is on high alert as it faces its first major heatwave of the summer, with temperatures expected to climb as high as 42C.
Aemet, Spain’s national weather agency, issued a special warning, forecasting highs of up to 42C in the country’s southern regions in the coming days.
In neighbouring Portugal, around two-thirds of the country will be placed under high alert on Sunday due to extreme temperatures and the risk of wildfires. Lisbon could see highs of 42C.
In Italy, where cities such as Naples and Palermo are bracing for 39C heat, the regions of Sicily and Liguria have introduced bans on outdoor work during the hottest parts of the day.
