Warning temperature will hit 45C on Tube Richard Gardner/Rex Features

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TUBE chiefs are rolling out emergency measures to help keep passengers cool after the Met Office issued a severe heatwave warning.

• London battered by thunderstorms and hail over the weekend

• Record temperatures to hit the capital this week

The mercury is expected to surge past 33C (91F) in London with temperatures underground soaring to a dangerously high 45C (113F) on some lines.

Transport officials are moving in 36 giant mobile fans into ticket halls and publishing health advice for travellers.

However, Underground trains will not be fitted with air conditioning until next summer - leaving passengers struggling in sweltering conditions.

The UK is currently on amber level two of the Government’s Heatwave Plan, but the Met Office expects it could reach level four by midweek. That would mean the country faces a state of emergency with possible water shortages and strains on the health service.

It is the first heat warning issued by the Met Office since the system was introduced two years ago.

"The heat is especially dangerous for the very young, older people or those with serious illnesses"

The health advice for Tube passengers includes:

*Carry water on all journeys

*Do not travel underground if you are feeling unwell

*Alert Tube staff if you are struggling in the extreme heat

The temperatures in London and the south east will be hotter than anywhere in the country - above 18C (64F) even at night.

And the heatwave is expected to last until at least next weekend.

A spokesman for the Department of Health said: “The heat is especially dangerous for the very young, older people or those with serious illnesses.

“In particular, it can make heart and respiratory problems worse. In extreme cases, excess heat can lead to heat stroke, which can be fatal.”

A Transport for London spokeswoman said: "We advise people to look at our advice before they travel on the Underground.”