Mayor of London Boris Johnson has put a prototype of the new Routemaster bus through its paces.
Mr Johnson was at the Millbrook test facility in Bedfordshire, where the bus's handling, emissions and engine performance are being tested.
Transport for London (TfL) said the bus will be 40% more efficient than normal double-decker vehicles.
If tests are successful, the new Routemasters could be on the streets of central London by early 2012, TfL said.
Mr Johnson said: "I am thrilled that we are now able to put this lean, green and thoroughly ravishing machine through its paces.
"I hope that the [testing] process will confirm we have built a real eco-warrior of a bus that can contribute to improving the air of our city, while transporting Londoners in great style and safety."
In January, a disability charity criticised the accessibility for wheelchair users, but TfL said after discussions with relevant groups, that the bus's design had been updated to improve access.
In December 2005, the original Routemaster was withdrawn from general service on accessibility and environmental grounds.
A competition was then started in 2008 by Mr Johnson to design a new Routemaster bus for the capital, and the winning entry was unveiled in May last year.