The Met Office said an unprecedented UK high of 39C was likely in England.

LNER has urged passengers not to travel as some trains will be cancelled and others will be delayed by speed restrictions forced by the heat to prevent tracks buckling.

The record is 38.5C, set in Kent in 2003, with the July record 36.7C.

North of the Border, 29C is expected around Glasgow and 28C in the west Highlands and around Dumfries.

However, the weather agency has also issued a yellow “be aware” severe weather warning for thunderstorms sparked by the heat from 3pm until 4am tomorrow.

The alert covers the eastern half of Scotland as far north as Aberdeenshire and much of England apart from the southwest.

A Met Office spokeswoman said: “Following a hot day (in places exceptionally so), scattered thunderstorms are likely to break out from late afternoon onwards.

“While some areas, particularly in the south of the warning area, may well avoid the storms, where they do occur they could produce 20-30mm of rain in less than an hour, along with frequent lightning, hail and gusty winds.

“Perhaps the main focus will be later in the evening and into the first part of the night across northern England and southern and eastern parts of Scotland, where storms could be more widespread and locally intense.”

LNER, the main Edinburgh-London train operator, said some services were likely to be cancelled as speed restrictions will be imposed between Peterborough and King’s Cross, which will also result in longer journey times.

Safety and operations director Warrick Dent said: “We apologise to customers for the inconvenience the forecast record-breaking high temperatures will inevitably cause.”

Virgin Trains, which operates Glasgow and Edinburgh to London services, said there would be speed restrictions between London and Rugby from noon to 8pm which may cause delays.

ScotRail and Caledonian Sleeper services were disrupted yesterday by lightning damaging signalling equipment between Perth and Pitlochry, and in northern England. Sleeper trains from Glasgow and Edinburgh to London were delayed by more than one-and-a-half hours, and those from Fort William, Inverness and Aberdeen by one hour.

The heatwave yesterday set new temperature records on the other side of the North Sea, of 39.9C in Belgium and 39.1C in the Netherlands, which could be broken again today.

Paris and other parts of France could see temperatures exceeding 40C, along with areas of Germany, Luxembourg and Switzerland.

Scotland’s Weather: Thunder and record temperatures threaten travel chaos