The original AEC Routemaster was used as the standard London bus type, with a rear open platform and crewed by both a driver and conductor.

After half a century, it was withdrawn from service at the end of 2005 (except for two heritage routes which operated until 2014 and 2019 respectively), in favour of a fully accessible one-man-operated modern fleet (including articulated buses), none of which featured a rear open platform.

The design for the new double-decker bus was inspired by the original AEC Routemaster and features three doors and two staircases to allow accessible boarding.

Unlike the AEC Routemaster, the new bus has a full front end rather than the protruding, bonneted "half cab" design, and a rear platform with a door that can be closed, rather than being permanently open.

The rear entrance initially had a platform and pole similar to the original Routemaster, with a door kept open for hop-on, hop-off operation when a conductor was on board.

From 31 December 2000, it became mandatory for all new buses delivered in the UK to comply with the Disability Discrimination Act 1995, leading to the development of the wheelchair-accessible low-floor bus.

The first Mayor of London, Ken Livingstone, supported the Routemaster during his first (2000–2004) term, indicating the type would be retained in a limited capacity on contract renewals as before.

Contributory factors to the withdrawal were said to be the risk of litigation over accidents arising from using the rear platform, the cost savings of one-person operation and the fact that passengers preferred the comfort levels of modern buses to the vintage Routemaster.

[17] Because of its single door (a serious drawback for a bus of this capacity) and continued mechanical problems associated with its unique design, the FRM was considered a dead end although it provided proof of concept.

Features of the new design included a side-mounted engine for maximum flexibility in door and seating layout and hydraulic drive to four small-wheeled axles for the lowest possible floor.

[19][20] London Transport had, by then, committed heavily to the Leyland Titan, to which they had significant design input and it was regarded as a more viable option.

[19][21] On 3 September 2007 the Conservative mayoral candidate for London, Boris Johnson, announced that he was contemplating introducing a modern-day Routemaster.

[22] Their design, dubbed the RMXL, was a hybrid technology low-floor bus with a lightweight aluminium space frame, with four more seats and twice the standing capacity of the old Routemaster, and operated solely by a driver.

This arrangement, through not requiring a mechanical transmission, allowed for a low floor and a step-free entrance into the lower deck from the rear platform.

The design was covered by the national press but attracted criticism from Livingstone as being too costly to justify and still not safe, despite proposals to monitor the rear platform with cameras.

[38] The body has two diagonal glass windows from top to bottom decks, one curving around the rear, the other on the right-hand side towards the front, which provides natural light to the interiors of both staircases.

These trials were curtailed due to the running-down and eventual closure of London Transport's bus Engineering Research department.

A static mockup was unveiled at Acton depot on 11 November 2010:[39] the first engineering prototype was driven by Boris Johnson at a public demonstration on 27 May 2011.

Within days of its unveiling, the first prototype was reported to have broken down on the M1 motorway north of London,[42] but this was due to human error; it had run out of fuel.

[44] However, Boris Johnson won the election and in September 2012 approved the order for 600 of the new buses, with public funding required estimated at £160 million.

[55] At the end of 2016, it was announced that London Mayor Sadiq Khan had discontinued procurement of the vehicles to save money and help pay for a promised four-year public transport fares freeze.

[56] No new Routemaster buses will be purchased for London, the funds instead going towards upgrading the city's existing fleet with the latest sustainable technologies.

[62][63] The Stagecoach Strathtay trial ended early in mid-December 2014, after the two vehicles proved incapable of running to the timetable and suffered a series of high-profile breakdowns in service.

[104][full citation needed] In 2020, it was announced that conversion so passengers only enter by the front door would be made permanent on the entire fleet, to reduce fare evasion which had been double that on other buses.

[53] To address issues caused by air-conditioning failures, in September 2015, it was announced a programme would commence to retrofit opening windows.

[110] The New Routemaster was also road-tested by Top Gear's James May in an episode where it left London and drove to attractions such as Cheddar Gorge before returning to the capital to take part in a 'Best of British' vehicle celebration.

[111] The New Routemaster has been criticised for the ineffectiveness of its air conditioning on hot days; the Mayor responded that the system was working as intended.

[112][113] The upper-deck windows have been criticised for being small, not giving comparable views to other bus models, and not letting in much light to the upper deck, making it "gloomy".

[118] London mayoral candidate and transport writer Christian Wolmar, who first revealed problems with the New Routemasters, said in July 2015: "This is further evidence that this project was misconceived from the start ...

[118] In one-person operation, as is the case on all units at all times after an early period with a conductor, the hop-on/hop-off advantage of the platform is negated by the closed door.