Thousands of train passengers could face severe delays until the end of the week - after "extensive" damage to overhead power cables.
The problems at Radlett in Hertfordshire mean services run by three train companies - Southeastern, East Midlands Trains and First Capital Connect (FCC) - are likely to be affected throughout today.
People could be delayed by up to an hour as there's major disruption expected on services between Luton and St Pancras International in London.
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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.