Hundreds of the Boris Johnson’s flagship New Routemaster bus are to be retrofitted with opening windows in order to end passenger complaints about “uncomfortable” and “sauna-like” temperatures inside the vehicles.

Instead of featuring opening windows in keeping with the rest of London’s bus fleet the new vehicle relies on an air-cooling system to control the internal temperature.

Since the bus entered passenger service in 2012 there have been persistent complaints about the effectiveness of the system, with passengers complaining on social media and to London Assembly Members about stifling heat and uncomfortable travelling conditions.

Transport for London has repeatedly denied suggestions that the cooling system is under-performing and insisted that temperatures inside the buses are “roughly” the same as on other buses.

Appearing before the London Assembly earlier this month Leon Daniels, head of surface transport at the agency, sought to blame the complaints on passenger “perceptions” caused by the lack of air travelling through the passenger saloon and the resulting absence of noise.

His remarks were openly mocked by London Assembly members, including Labour’s Tom Copley who told Daniels: “It is not just the sound of air, it is the fact that airflow itself does cool you down particularly when you have been sweating.”

Despite the denials it emerged in October 2014 that a new batch of 200 vehicles would have improved ventilation in a bid to reduce complaints and Mayor Johnson later confirmed he’d ordered TfL “to look again” at options “to improve comfort across the New Routemaster fleet.”

Windows will now be retrofitted to all currently in-service vehicles and come as standard on new vehicles at a cost of more than £2m.

The embarrassing u-turn follows the recent revelation that Transport for London doesn’t own the design of the new bus despite repeated past statements to the contrary and news that the rear platform will be closed on the majority of the buses because TfL will longer subsidise the cost of employing a second crew member to supervise its use.

Transport for London’s 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.

“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.”

Responding to the news, Green party Assembly Member Darren Johnson said: “Whilst I am pleased that passengers will now be able to open windows and get some respite from sweltering top-deck temperatures, I am angry that £2m of Londoners’ money will be used to go back and fix this problem.”

Labour’s Val Shawcross added: “With passengers enduring years of suffocating journeys on overheated buses, these upgrades will be welcomed. But the paying public has been left to fork out £2 million pounds for more upgrades to what was supposed to be a state of the art vehicle.

“It’s clear the Routemaster has come to represent yet another ill-thought out, increasingly expensive ‘legacy’ project for Boris Johnson to leave behind.”

And Caroline Pidgeon, Leader of the Liberal Democrat group on the London Assembly, said: “Anything to make travelling on the new routemasters more bearable during the Summer is obviously welcome. These buses are quite frankly a cauldron on wheels during hot weather.

“However the Mayor must now explain why the needs of passengers were overlooked when he decided to purchase these incredibly expensive buses. It simply beggars belief that such expensive buses were ever purchased by the Mayor and TfL with such a basic design fault.”