Trains on the District line will be air conditioned by 2010, London Underground has revealed.

As well as the District line, which travels through Putney, Wimbledon and Richmond, other partly above-ground lines such as the Circle and Metropolitan lines will also be fitted with air conditioning.

The plans come as part of London Underground's proposals to keep the Tube network cool in hot weather, which remains a challenge on deeper lines such as the Northern and Piccadilly lines.

About 40 industrial-sized fans will be installed at stations including Charing Cross, Bond Street and Bank this summer.

In addition, a large number of station ventilation shafts and fans are being brought back into service.

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."