Boris Johnson has insisted Northern Ireland businesses, farmers and fishermen have nothing to fear from Brexit and everything to gain.
The Mayor of London made the case for leaving the EU as he visited the Wrightbus factory in Antrim to officially announce a Transport for London order for 200 more of its famous Routemaster red buses.
The deal, which is worth £62 million, will sustain 300 jobs at Wrightbus and will take the number of Routemasters on the streets of London to 1,000.
Mr Johnson's 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 on Monday.
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."
He added: "We need to lift our eyes to the horizon, we need to think globally. The European Union, of course it's a hugely important market for us, but as a share of world trade it is diminishing and they are going in a very different direction from the rest of the world in the sense they are trying to create this political union based around the euro which is not really where we want to go."
The London mayor 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."
Northern Ireland Secretary Theresa Villiers, another senior figure in the leave EU camp, accompanied Mr Johnson on his visit.
Stormont First Minister Arlene Foster, whose Democratic Unionist party is also recommending a break from the European Union, and Sinn Fein Deputy First Minister Martin McGuinness, a strong supporter of the link with the EU, were among local political representatives in attendance.
Mrs Foster welcomed the latest Transport for London deal.
"Wrightbus is a shining example of a successful indigenous Northern Ireland business," she said.
"Innovation is a key element of economic success and Wrightbus has once again demonstrated their manufacturing credentials.
"This latest contract not only results in almost 200 Routemaster buses servicing London but it is a timely boost for the Northern Ireland manufacturing sector."
Mr McGuinness added: "Wrightbus is a flourishing business and this latest contract puts them in the manufacturing fast lane.
"The success in winning contracts reflects the sharp Wrightbus focus on innovation and marketing which has resulted in it becoming a major player on the global market.
"This follow up order is testament to the quality of the hard work and skill of their workforce and the benefits to our economy will extend beyond the gates of Wrightbus."
The Wrightbus Group has a total workforce of 2,000.