More London bus orders could be in the pipeline for a Northern Ireland manufacturer, it was revealed.
London mayor Boris Johnson hailed the double-decker as one of the greenest in the world during a visit to the factory where the fleet is being built in Co Antrim and urged greater investment in public transport.
The Commissioner of Transport for London said there could be more orders for Wrightbus as the English capital takes delivery of 600 of the characteristic red vehicles over the next three years.
Sir Peter Hendy said: "These vehicles are so popular, I am sure there will be more of them. We think they are fantastic."
He joined Mr Johnson on a tour of Wrightbus factories in Ballymena and Antrim, watching work including the fitting of windscreens.
The mayor said a million more people would come into London by 2021.
"We have got to keep investing in capacity, we will be making our case (to the Treasury) in the normal robust way."
He added: "We cannot cut into the bone and sinew of London's transport infrastructure."
It takes around eight weeks to build a bus. The contract with Wrightbus is worth around £200 million.
The buses can achieve fuel efficiency of around 11 miles to the gallon.
Mr Johnson said: "It is a beautiful thing, it is beautiful for everybody in London, they know they are in the presence of an ascetic masterpiece, a wonder of British design.
"It is a real sign of the confidence that we have in British manufacturing that we have now delivered a bus that is frankly knocking spots off every other green bus around the world."
He said the buses are charismatic "mega fauna" for London which help to drive employment around the country.
He quipped: "The urea that you use to purify the vapours from this bus comes from contented British cows or possibly human beings."
Mr Johnson went on: "It is brilliant for passengers who have the sovereign right as free sentient adults to hop on and hop off which was taken away by the Brussels fanatics."
Northern Ireland Secretary Theresa Villiers said today was a huge success story for Wrightbus.
"The fact there are going to be 600 of these buses running off the production line is a real testament to the quality of Northern Ireland's engineering. Wrightbus are a class act."
The order will see the workforce at the new chassis plant more than double from 40 people to 90 when full production gets started.
The contract will also safeguard 220 jobs and sustain 18 apprenticeships.
The new vehicle is the greenest diesel-electric hybrid bus in the world, producing 20% less carbon dioxide then the fleet average hybrid bus, Transport for London said.
As well as the manufacture of the chassis and superstructure in Northern Ireland, a number of components are made by companies from around the UK.
Eight of them are already in use in London and by the end of June, route 24, which passes Parliament and Trafalgar Square, will use the new vehicles exclusively.
Each bus costs around £354,500 and has an estimated lifespan of 14 years.