Scotland Yard said two passengers still remain trapped on the bus and two uniformed police officers who were travelling on the bus, are among the 30 injured.
London Ambulance Service say 6 people are seriously injured.
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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.