Parts of London's Tube network could be shut down on very hot days to prevent "loss of life", the mayor has warned.
Passengers endure extreme temperatures in the carriages
As temperatures inside carriages rose to well above 30C on Monday, Ken Livingstone admitted climate change may force drastic measures in the future.
A groundwater cooling system to combat hot weather is currently being tested.
Mr Livingstone said: "We might face, in a few years time, having to shut some Tube lines on particularly hot days just because it won't be safe."
He added: "You reach a point where the underground will become literally intolerable and you could face the prospect of loss of life."
Engineers are working on a concept of taking ground water up through stations to cool them and using the heat to power homes and offices above and bring the temperature down.
Mr Livingstone said it was a race against time and the authorities would have to think about closures "if the temperature goes up faster than we fear it's going to".
Last month about 1,000 passengers were trapped for nearly two hours when three trains were held up in a tunnel.
London Underground said there were no plans to close any Tube Lines during the summer.