The Wrightbus factory in Antrim has secured a Transport for London order for 200 more of its famous Routemaster red buses.
The deal, which is worth £62m, will sustain 300 jobs at Wrightbus and will take the number of Routemasters on the streets of London to 1,000.
London Mayor Boris Johnson hailed the new deal with Wrightbus as he toured the factory floor.
Clad in a high-vis jacket, at one point he swung from under the chassis of one of the vehicles to demonstrate how robust the structure was.
"This is fantastic news for London because we are going to get another 200 of these superb Northern Irish-built new buses for London, Routemaster buses," he said.
"I hope it's good news also for the Northern Ireland economy. What it shows is how keeping London moving keeps Northern Ireland moving, keeps the UK moving together. I am very pleased today to see we are going to have 1,000 Wrightbuses on the streets of London - 1,000 new Routemasters."
Mr Johnson made the trip to the factory, while also campaigning for a so called Brexit.
His visit to Northern Ireland came 48 hours after Prime Minister David Cameron travelled to the region to make the case for staying in the EU.
The Tory MP, who is one of the vote to leave campaign's most high-profile members, unsurprisingly made a very different pitch today.
Asked about concerns voiced by some senior business figures in the region, Mr Johnson said: "I think there is absolutely nothing to be concerned about, indeed everything to gain.
"We have a real opportunity to get out from under the encumbrance, that constricting force which is the European Union and the bureaucracy that's involved."
He also claimed there would be benefits for the farming and fishing industries.
"I think for Northern Ireland it would be good news from the point of view of fisheries. I think the farming community, the subsides would be better tailored to their needs and, as for exports around the world, we should be so positive."
