New air conditioned trains, which have been running on the Metropolitan line for the last couple of years, are now being introduced on the Hammersmith & City line and later this summer on the Circle line.
The two lines will be fully served by the new trains by the end of next year and, by 2016, an air conditioned service of 191 trains will be in operation covering 40 per cent of the Tube network.
For people using two of London's busiest Tube stations, Green Park and Oxford Circus, station cooling systems that were installed last year have made a significant difference by reducing the temperature at platform level say TfL.
Other work to improve ventilation shafts, restore out of service fan systems and portable fans will also be helping to keep passengers cool.
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The most hotly-tipped date is July 13, following the Nato summit which the president is due to attend in Brussels on the previous days.