Transport for London is to fit opening windows on the upper decks of Thomas Heatherwick's new London buses as the designer originally intended, after passengers complained about the heat.

Heatherwick welcomed the move and is working with TfL and manufacturer Wrightbus on the amendments.

"The studio welcomes TFL's decision to incorporate opening windows in the buses," said a spokesperson for Heatherwick Studio.

The sealed upper windows on 550 operational buses will be replaced with windows that can be opened – at a cost of £2 million. A further 250 buses that are on order will be fitted with the new windows.

Heatherwick initially wanted natural ventilation on the buses, but was overruled at the design stage by TFL.

TFL director of buses Mike Weston said: "We were aware of passenger concerns about the cooling system on the New Routemaster and the mayor asked us to work with Wrights to look at possible design options to improve passenger comfort."

He added: "We're pleased they've now been able to come up with an affordable and working design to install opening windows, which we anticipate will have been installed across our entire fleet by next summer."

Temperatures on the upper decks have reached 30 degrees Celsius and more, leading to protests from passengers.

Heatherwick's design for the replacement of the iconic Routemaster was unveiled in 2010, with the first buses becoming operational in 2012.

The design retains the open rear platform of the Routemaster, which allows passengers to jump on and off, while introducing features including wheelchair access, air conditioning, energy efficiency and greater seating capacity.

It also features three doors compared to the original's one.

"The key aspects of the brief were to make a bus that uses 40 per cent less energy than the existing diesel buses and to improve the reliability," Heatherwick told Dezeen in an exclusive movie.

"The romantic bus that people love and think of in London has one door, and that means that you are waiting for all the passengers to unload before all the passengers at the bus stop can load up."

The original diesel-powered Routemaster first went into service in the 1950s but was finally withdrawn in 1995, when it was replaced first by single-deck "bendy buses" and later a variety of double-deck vehicles, none of which earned the affection of the London public.