Scotland is expected to have its hottest day of the year later, with temperatures forecast to hit 30C in southern areas.

It will meet the official criteria for a Scottish heatwave, as the UK swelters through some of its hottest June nights on record.

In the Borders, gritters have been deployed to spread sand over the main roads to try and reflect heat and cool the surface.

The Met Office stipulates that temperatures in Scotland must reach 25C for three consecutive days, in the same area, for it to be classified as a heatwave.

A high of 29C was recorded at Dyce in Aberdeen on Tuesday, beating the year’s previous high of 28.1C at Floors Castle in the Borders on 25 May.

Scottish Borders Council said the extreme heat was damaging roads in the area, with some reaching 45 degrees.

The B6363 Lauder to Stow road is already closed because of heat damage.

David Robertson, chief executive at Scottish Borders Council, said they were expecting it to reach 31C in Newcastleton and Hawick.

He said: “If you see the gritters, it’s not the council being daft, it’s us actually trying to protect the road surface.

“We’re providing a fine layer of sand to prevent the melting of the tar.

“What we’re seeing is ambient road temperatures at the height of the heat up in the 40s, and that causes the tar on the roads to melt.”

Meanwhile, Dumfries and Galloway Council has asked residents to “spare a thought for our frontline staff” wearing personal protective equipment in the heat.

Police Scotland issued reminders about water safety and the risk of drowning, external from “cold water shock”.

BBC Scotland weather presenter Gillian Smart said the heat would peak as southeasterly winds pull in intensely hot, humid air from the continent.

She said: “The highest temperatures will be in central and southern Scotland, where we’re likely to see 30 Celsius.”

However, she warned of a “thundery breakdown” on Thursday afternoon.

“A weather front to the northwest will interact with the very warm air to produce heavy, thundery showers across the Western Isles, the Northwest Highlands and the Northern Isles.

“These thundery showers will feed southwards across the country, torrential in places, and bringing a risk of hail.”

A yellow warning for thunderstorms has been issued for Highlands, Orkney, Moray and Argyll and Bute from 15:00 to 23:59, and for the rest of mainland Scotland from 00:00 to 23:59 on Friday.

Parts of England and Wales have a rare red warning for extreme heat in place and it is expected to be another scorching day for most of Europe.

Temperatures will once again be around 12 to 15C above average across northern Spain, France and into Belgium and the Netherlands.