Experts on bus design and production have cast doubt over Boris Johnson's pledge to build a new Routemaster if he is elected mayor of London on May 1.

In his transport manifesto, the Tory candidate pledges to scrap London's 340 articulated buses and launch a competition to design a new Routemaster. But experts interviewed by Transit suggested that it was a promise he could not deliver. A senior executive with a UK bus builder said a new Routemaster could not be certified, and would be too expensive. "It's not been thought through," he told Transit.

Other experts also cast doubt on the plan. Sandy Glennie, the former managing director of Volvo Bus UK, said that having an open rear platform on a new bus was a "non starter". Andrew Braddock, the former head of access and mobility at Transport for London, estimated that the vehicles would cost the mayor more than £250,000 each, much more expensive than a conventional double decker or an artic.

London mayor Ken Livingstone said his rival's plans for new Routemaster buses could kill more than 10 people a year through passengers falling off the back of them. In an ITV London debate, the mayor warned that the plans would never get off the drawing board because no manufacturer would want to be sued over deaths or injuries caused.

But Johnson hit back, claiming that his new design would be safer than the articulated buses they would replace.

Johnson's campaign team have meanwhile denied claims that he is set to sack senior management at Transport for London.