A disability charity says the new Routemaster bus (pictured) is "completely inaccessible" for users of large wheelchairs.
The bus is being built in Northern Ireland, with full scale mock-ups already being tested.
But pressure group Transport for All, which was able to test the bus, wants more space for wheelchairs.
Transport for London said the new bus had more than the legally required space for wheelchairs.
But wheelchair user Yousef Bey-Zekkoub, of Transport for All, said: "I went to visit the new bus and I was completely disappointed.
"The wheelchair space is even smaller than on the bus I use regularly.
"It is completely inaccessible for electric wheelchairs."
Mr Bey-Zekkoub claimed one woman trying to get on the bus in a recent testing session could not get on board at all.
He continued: "With the 2012 Olympics TfL has the opportunity to show the world what an accessible bus is.
"Buses should be accessible for everybody, not just the able-bodied."
Mike Weston, TfL's operations direction for London buses, said: "Since the mock-up was delivered last year, we have been holding sessions with user groups who have been trying the vehicle.
"Based on them we have made some changes to the wheelchair bay."
Mr Weston revealed that several rails and seats had been moved, but insisted they could not remove seats altogether as elderly people needed plenty of seating on the lower level.
He added: "We have now got something better than a lot of buses we have already got, and way in excess of legal requirements."
The first five buses are due to enter passenger service in early 2012.
Tuesday, February 1, 2011
Posted by BA Haller at 6:23 PM