The London Underground has not introduced any new air-conditioned Tube trains for nearly nine years, spanning almost the entirety of Sadiq Khan's tenure as Mayor of London. The issue has come into focus as temperatures in the capital are forecast to reach 34C, creating severe conditions on the network's deep-level lines, and with the National Heat Risk Commission describing heat-proofing the Tube as an "economic necessity". The last new air-conditioned Tube train was brought into service in June 2017, a year after Mr Khan took office.

At present, 190 Tube trains have air conditioning, but the deep-level lines that make up 60 per cent of the network still lack it. Transport for London says the narrow Victorian-era tunnels leave little room to fit cooling equipment on the outside of carriages, and engineers fear expelled heat could make platforms dangerously hot. However, cooling deep lines is not impossible: the Piccadilly line is due to become the first to be fitted with air conditioning later this year, using new Siemens-built trains with compact units under the carriages, though these are not expected to be ready until at least December. The Elizabeth Line, opened in 2022, is air-conditioned but is not part of the Underground.