But the Standard has discovered that the five buses will be delivered to the capital this year after a boat trip of at least 17,240 miles.
The vehicles themselves will have been built only 350 miles away from London in Ballymena, Northern
Ireland, but will then be shipped at least 8,620 miles to San Diego in California for engines to be installed.
Following this the buses, which will be used on the Covent Garden to Tower Gateway route, will make the long journey back to Britain to be finished before they are delivered to TfL in November.
Because a single container ship emits more pollution than 2,000 diesel trucks and uses 30 times more fuel per mile than a car, critics have questioned just how eco-friendly the project really is.
However, sending the buses by boat rather than as air freight has reduced the carbon footprint of the trip.
Richard Bourn, of the Campaign for Better Transport, said: “To ship these buses such a long way seems absurd. “We would also question the use of hydrogen buses in the first place, as producing the hydrogen for them is not efficient.”
Liberal Democrat leader at City Hall, Mike Tuffrey, said: “Their cost and their surprisingly large carbon footprint they will generate before arriving in London raises some basic questions as to whether they are a good deal.
“The Mayor has some serious questions to answer as to whether £5 million could be better spent on low-tech measures now that would improve fuel efficiency and reduce pollution from London's existing buses.
“Advances in new technology are often expensive, especially in the early days of becoming established, but I am simply staggered by reports showing how expensive these buses are turning out to be.”
The buses are being delivered by ISE, a US firm. TfL signed a contract with it in 2007, instigated by former mayor Ken Livingstone, but the project has been continued by Boris Johnson despite major budget cuts at TfL.
Transport bosses say water is all the hydrogen-fuelled bus will release into the air, and Mr Johnson has justified the buses as important because they will help to reduce CO2 emissions from transport and tackle climate change in London.
A TfL spokesman said: “Hydrogen buses are cutting edge technology and there are currently only a very limited number of specialist suppliers available.
“ISE were selected as our preferred supplier, with Wrightbus appointed as the bodywork supplier.
“While there is an increased carbon footprint for the initial buses, our long term programme will ensure the market for hydrogen vehicles is stimulated and that footprint will reduce considerably.”