Over-running engineering works are causing disruption near London Bridge.
Because of this, there are delays of up to 80 minutes to journeys between New cross and London Bridge.
These delays will continue until further notice.Trains between Dartford and London Bridge via Greenwich may be diverted via Lewisham.
First Capital Connect services are currently unable to call at London Bridge.
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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.