Police, firefighters and ambulance staff are currently at the scene of John Lewis in Bluewater Shopping Centre which has been evacuated. It is believed the chemical spillage was from a fridge in the department store.
There are unconfirmed reports of injuries as a result of the leak.
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The high of 35C was recorded on a Bakerloo line train between Piccadilly Circus and Oxford Circus at 2pm.
Weather forecasters say temperatures could reach 35C, beating the previous record of 34.6C in the scorching summer of 1976.
After years of political manoeuvring over the future expansion of Heathrow Airport, there has been an important development this lunchtime.