Whatever you do, don't get on a Routemaster bus in London today

London's new Routemaster buses may be cool in a design sense, but on the hottest day of the year, they're the exact opposite - temperatures on board could exceed 60 degrees Celsius.

The Routemasters don't have windows on the upper deck because they're fitted with an air cooling system - but as passengers will attest, it's not very effective at the best of times.

In the summertime the lack of ventilation turns the bus into a mobile sauna, especially since the third door at the back of the bus is often kept closed if the driver doesn't have the help of a second member of staff.

Today temperatures in the capital are expected to peak at 33C, making it likely that temperatures inside the buses will be much higher than that - if it's 32C outside, after just half an hour in a closed car the temperature can exceed 51C.

The legal limit for transporting livestock is 30C, but no such rule exists for human commuters.

Spokesperson for Transport for London Dan Maskell told i100.co.uk that all buses going for repainting are getting white-painted roof panels to reflect heat, and new buses must have insulated reflective roof and side panels, as well as tinted side glass.

"Buses cannot regulate temperatures in the same way as coaches or trains," he said. "Because of the nature of bus journeys, cooled air will be lost from the vehicle during the frequent intervals at which it stops."

The new Routemasters use a diesel-electric hybrid motor which produces around half the amount of carbon dioxide of a conventional bus, and adding air conditioning units would dramatically increase its carbon footprint.

So, if you have to take a Routemaster today, there's a small comfort in knowing that your discomfort is helping to stop the planet get hotter than it already is.

More from The Independent: UK Heatwave: Temperatures on the Tube are going to exceed the legal limit for transporting cattle