The Tube carries more than a billion passengers a year
Parts of the Tube network will have air conditioned trains from 2010, London Underground has announced.
The move would see trains on "surface" lines such as the Circle, District and Metropolitan lines equipped with air conditioning in about two years.
Industrial-sized fans will be used to improve station air flow this summer.
But transport bosses said cooling the Tube remained a major challenge, especially on deeper lines such as the Northern and Piccadilly lines.
About 40 fans will be installed at stations including Charing Cross, Bond Street and Bank.
Some tourists refuse to use the Underground
In addition, a large number of station ventilation shafts and fans are being brought back into service. So far 50 such fans have been renewed.
A scheme in which pumped-out water is used to cool platforms at Victoria Tube station is being repeated this summer.
London Underground managing director Tim O'Toole said cooling the Tube was "a major and long-term engineering challenge".
"The London Underground's Victorian heritage is its charm and its burden," he said.
"Today the Tube carries over a billion passengers a year and as passenger demand continues to grow, so does the need to increase capacity and upgrade the network."