Britain has sweltered for a third day running as temperatures climbed to 26C and parts of the country saw “barely a cloud in the sky”, forecasters say.

Trailing the hottest April day for 70 years, which peaked at 29.1C in London, Friday continued the trend of unseasonably warm weather.

St James’s Park in London reached 26.1C – more than 10C above April averages – but heavy thunderstorms threaten to dampen weekend plans for some.

The Met Office said showers will sweep in from south-west England into the Midlands over the course of Saturday, escalating into hail storms and even surface flooding by the time it reaches Yorkshire in the afternoon and evening.

Elsewhere, the warmth is likely to survive, menacing tens of thousands of runners as they prepare for the London Marathon.

Meterologist John West said: “Broadly, it has been looking very sunny.

“We have had a little caveat to that in western Scotland, there has been some showery rain, some more cloudy conditions spreading across the far north west and also the west coast of Wales, down towards Devon and Cornwall have seen quite a lot of fog as well.

“Across England and Wales, there has been hardly a cloud in the sky and temperatures responding really quite nicely to the sunshine.”

A man sits in the sun at Greenwich Park, London (Victoria Jones/PA)
A man sits in the sun at Greenwich Park, London (Victoria Jones/PA)

The South East was the warmest part of the country and will continue to be across the weekend, he added.

Exeter enjoyed 20C on Friday, while Wiltshire hit 23C and central Wales enjoyed highs of around 20C.

Northern England reached between 15C and 18C.

But in Aberforth, Wales, fog ensured a tepid 8C had not been exceeded by mid-afternoon. In western Scotland, it was similarly cool, at around 11C or 12C.

All four home nations enjoyed their hottest day of the year so far on Thursday, with everywhere seeing a high of at least 20C (68F).

The hot weather is a result of warm air moving up from the Azores in the south.

The London Marathon on Sunday could be the hottest yet, with the 1996 record of 22.7C (72.9F) appearing to be under threat.

Mr West said: “There is going to be an awful lot of looking at the radar and the satellite on the day, because it does look as if there are going to be some showers around first thing in the morning, so the earlier you start, the better, I think.

“It does look as though it is going to be rather humid – although not quite as warm as we have seen over the last couple of days.

“We are likely to see highs of around 21C or 22C which would be close to the record.

“Also the UV, the pollen and the pollution levels are going to be high.”

Race organisers have announced they will add more water, ice and shower stations along the 26.2-mile route.

Conditions may be especially difficult for fancy-dress runners, including the almost 100 attempting Guinness World Records dressed in outfits like a suit of armour, a Paddington Bear costume and ski boots.

Regular runners are being advised to consider dropping their goal times and to run more slowly.