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London Underground has asked contractors to suggest a way to cool the Tube system in the capital.

It is understood that Costain, Balfour Beatty and Morgan Est, the engineering arm of Morgan Sindall, have been invited to tender for the work.

Transport for London says the average temperature on the Tube in London during the winter months is 25掳C, rising to 31掳C in the summer. It concedes, however, that it can get a lot hotter.

On 19 July this year, The Daily Telegraph carried a report that the temperature had reached 47掳C on the previous day, and quoted a medical expert who said the human body 鈥渟tarts to cook鈥� at that temperature.

The latest attempts at bringing the temperature down are part of a wider programme under which Transport for London intends to invest 拢10bn over five years to improve the capital鈥檚 transport network. More than half of that has been earmarked for the Tube.

Cooling the Tube tunnels will be a tough challenge. The London Underground is the oldest system of its type in the world and its basic tunnel infrastructure has changed little since the first lines were constructed more than 140 years ago.

The tunnels were built with sufficient room only for trains. Air-conditioning had not been invented so no provision was made for its installation.

The methods under investigation could include a fanning the tunnels, or installing a groundwater cooling system.

A spokesperson for London Underground declined to comment on the specific companies that it was working with, but said: 鈥淲e are working with a range of organisations on a whole range of potential solutions to cool the Tube.鈥�

He emphasised that talks were at an early stage.

Contractors鈥� bids are part of a feasibility exercise that is intended to lead to the development of prototype systems.