Sweltering Britain is soaring towards what could be the hottest day ever, as trains are delayed to prevent tracks from buckling in the heat and reservoirs run dry.

The Met Office said Friday could be the UK's warmest July day as temperatures are expected to rise to at least 37C (98.6F), with the possibility the all-time heat record of 38.5C (101.3F) will be broken.

But thunderstorms are expected to provide slight relief from the scorching heatwave. After weeks without rain, torrential downpours are due to hit parts of Britain from Friday afternoon, with as much as 30mm (1.2in) expected to fall in just an hour.

With a chance of hail and strong winds alongside the torrential downpours, the temperature could drop back to the high 20s, the Met Office said.

Temperatures reached a high of 32C on Wednesday as the warm weather continued in parts of the UK.

Wisley, Surrey, was the warmest place in the UK, while the lowest temperature of 4.2C was recorded in Braemar, Aberdeenshire.

A high of at least 37C (98.6F) is forecast for London and the south-east on Friday.

An amber "heat health watch" warning remains in place for parts of England, with people being warned to try to stay out of the sun between 11am and 3pm.