Eight of the new vehicles will eventually run on route 38 between Victoria Station and Clapton Pond, though just one bus will enter service today.
The new bus was a 2008 manifesto commitment by Johnson who has previously hailed it as “the latest, greatest masterpiece of British engineering and design”.
Speaking on Monday the Mayor said: “The green innards of this red bus mean that it is twice as fuel efficient as a diesel bus and the most environment-friendly of its kind. When ordered in greater numbers it will make a significant economic contribution to the manufacturing industries, while also helping deliver a cleaner, greener and more pleasant city.”
In October TfL’s surface transport director Leon Daniels told the London Assembly the bus needed “thousands of miles” of in-service testing and would not go into full production until at least the Summer.
The vehicle was meant to start service last week but was delayed after certification by the Department for Transport took longer than expected.
Opponents have criticised the Mayor for spending over £11m on the project while implementing a series of fares increases.
Caroline Pidgeon, Leader of the Liberal Democrat group on the London Assembly, said: “Boris Johnson made two specific promises to Londoners.
“One that the new ‘routemaster’ style buses would be on London’s streets in 2011, and two, that the development costs associated with any new bus would be picked up by industry. He has broken both these promises.
“What Boris Johnson has managed to achieve is to hike up the cost of a single bus fare by 50% and ensure that buses on routes across London are far more crowded.”
Labour MP David Lammy has written an open letter to the Mayor questioning the cost of the new bus.
In his letter Lamy says: “Should the full number of these new buses finally reach the road, it will still only be eight buses out of a fleet of 7000 across London, at a cost of £1.4million per bus.
“That compares to the price of a conventional double-decker of around £190,000. Riding this bus is surely the most expensive bus ticket in history.”
The new bus includes three door entry, a feature the Mayor and Transport for London have previously blamed for high fare evasion levels on the bendy bus.
Despite this the conductors on the new vehicles will not be responsible for collecting and checking fares. Instead TfL “will require all Oyster card holders to touch in”, a move the Mayor has said would “reduce the perception of fare evasion on the vehicle”.
