The old Routemaster was inaccessible to wheelchairs and pushchairs
A contest to design a new version of the iconic Routemaster bus has been launched by London's mayor.
The jump-on, jump-off double-decker bus was withdrawn from regular service in the capital in December 2005.
Mayor Boris Johnson was at the London Transport Museum to invite designers to submit ideas, with the best entry winning a first prize of £25,000.
But trade union Unite said the mayor could not introduce a new bus "without raising fares substantially".
Mr Johnson wants the new bus to have a stylish, imaginative design, which will have a big impact on London's streets. The idea to reinvent the iconic bus was one of his election pledges.
"London deserves a bus fit for the 21st Century and I want as many people as possible to submit their ideas," he said.
Key features designers must consider include an open platform to allow passengers to get on and off quickly and easily, a good use of interior space, disabled access and green technology.
Routemasters were phased out from regular service by the end of 2005 as they were inaccessible to wheelchairs or pushchairs.
The bus has now been restricted to operating additional services on popular tourist routes.
Mr Johnson also made an election promise to phase out bendy buses, saying they were "never suited to London's roads".
No new bendy bus contracts will be renewed, Transport for London said on Friday, with all bendy buses due to disappear from the capital's roads by 2015.
But the Unite union said Mr Johnson had underestimated the cost of replacing bendy buses with a new bus.
Regional officer Peter Kavanagh said: "You would have to be Merlin the magician to make his figures add up."
Mr Kavanagh said that "on labour costs alone" it would cost £135m to replace bendy buses.
But the mayor's spokesman said it was "too early to say" how much the new buses would cost.
In April Mr Johnson said it would cost "around £100m" to replace bendy buses with new Routemasters.
The bus design competition closes on 19 September 2008.