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Temperatures soared to more than 34C in the capital on Wednesday – making it the hottest June day since 1976 – as ambulance calls were up by 46 per cent.

The mercury peaked at a staggering 34.5C in Heathrow, which also made it the hottest summer solstice on record – outstripping the previous record for the longest day of the year when 31.7C was recorded in Camden Square in 1936.

Meanwhile between 2pm and 3pm on Wednesday afternoon the London Ambulance Service received 362 calls compared to 248 at the same time last week.

Ambulance chiefs warned people to only call for life-threatening or serious emergencies and to check on vulnerable friends, family members and neighbours such as older people who are more at risk.

Medical Director Dr Fenella Wrigley said: “We’re extremely busy and I’d ask Londoners to only call us if it’s a medical emergency.

"Those with more minor conditions should call NHS 111 or go to their GP, or local walk-in centre or pharmacist.”

People are also advised to drink plenty of water, stay in the shade to keep cool, carry essential medication with them, cover up and drink alcohol in moderation.

The five day heatwave has seen each day since Saturday top the last for the hottest day of the year, but temperatures are set to fall on Thursday.

A weather warning has also been put in place for the morning, with torrential rain set to hit parts of London.

Met Office forecaster Emma Sharples told the Standard: “In the London area tomorrow we are looking at temperatures of 26 or 27C but we will also see the risk of some heavy showers.

“There is a weather warning in place as we could get rumbles of thunder and some real downpours, which could quickly lead to localised surface water flooding.

"There is a risk of hail as well.”

Pollution warnings have been triggered across the capital’s transport network with emergency alerts displayed at bus stops, on road signs and on the underground as the mini-heatwave entered its fifth day.

Air quality and smog were expected to be at their worst near main roads at around 4pm today with asthma sufferers, children and elderly people warned to avoid the most polluted routes.