Speed restrictions are again expected to be imposed on most Tube lines as the heatwave continues.
Engineers have been checking the track temperature every hour
On Wednesday a 25 mph speed restriction was introduced on the Metropolitan Line, the District Line was running at 20 mph and the Jubilee, Piccadilly, Central and Northern lines were also affected.
The London Eye was closed on Wednesday
after temperatures in the pods became too uncomfortable.
And lifeboat crews are urging people attempting to cool off in the River Thames to be cautious.
The Royal National Lilfeboat Institution warned that swimming in the river is dangerous and any water activities should be undertaken as part of an organised group.
Forecasters said temperatures in London reached 35C on Wednesday - making it the city's hottest day ever on record.
It is predicted it will be a degree or two lower on Thursday but London transport bosses are closely monitoring temperatures.
Engineers are going out on the hour every hour to check the levels.
When the air temperature goes above 33C or the track temperature is 49C then a 30% speed restriction is imposed.
And if the air temperature reaches above 36C or the track temperature hits 53C then a 50% speed restriction is introduced.
Tigers at London Zoo were fed lollies made of frozen blood
Early morning swimmers have been enjoying the warm weather by taking a dip in the Serpentine Lake at Hyde Park.
One told BBC London: "The water feels very refreshing, it's certainly quite warm at the moment and the hottest we've had this year."
At London Zoo the tigers were finding the heat too much and refused to feed apart from tasting some alternative lollies.
Zoo keeper Nicki McKay told BBC London: "They [tigers] do like the heat but obviously it's been very hot over the last few days.
"So we are giving them some ice lollies which are made from frozen blood. We use real blood."
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