Britons have been warned not to "underestimate the heat" on what could be the hottest day of the year amid concerns that it will become "highly dangerous" to be outside.

The UK is expected to see record-breaking temperatures on Friday, with the mercury set to rise to 37C in London and the south-east, the second day of a four-day heatwave.

Forecasters said it could even surpass the 37.8C recorded in Heathrow on July 31 – the hottest day of the year so far and the UK's third-warmest ever.

But the Met Office warned that, after a slightly cooler week, people could underestimate the heat, which could cause dehydration and sunburn, especially among the most vulnerable.

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