The UK is “likely” to have its seventh consecutive day of 30C heat in September, the Met Office has said.
It comes as a further weather warning has been issued, covering southern Scotland from midnight until 6am on Monday morning.
This follows thunderstorm warnings already in place for parts of the UK on Sunday afternoon.
The heatwave has already broken the record for the most consecutive days with temperatures above 30C in September, with Saharan dust generating vivid sunsets and sunrises in the clear conditions.
Met Office meteorologist Tom Morgan said that Sunday is set to mark the seventh day of 30C weather, with the highest temperatures expected in the north Home Counties.
“It’s looking likely that we will see the seventh day of temperatures running at 30C or higher, which is unprecedented.
“We have never seen anything as long lived in terms of a heatwave in September before.”
Amid the hot weather, a yellow thunderstorm warning spanning Northern Ireland, northern parts of England and Wales as well as southern and northeastern Scotland has been issued by the Met Office from 2pm until 11.59pm on Sunday.
The warning means some people could be in store for flash flooding, lightning strikes, hail or strong winds, with possible interruptions to road access and public transport if such circumstances were to occur.
A similar weather weather warning will continue to cover southern Scotland from midnight until the early hours of Monday morning.
“We have a third thunderstorm warning overnight for southern Scotland,” Mr Morgan said.
“That expires at 6am.
“And then once that’s cleared, there will be a few thunderstorms tomorrow.
“But for the vast majority they will be a bit more scattered in nature than today.”
