The Belfast-based company Whitemountain Quarries has signed a five-year deal with the road-building company Eurovia for the supply of gritstone from its quarry at Ballystockart on the outskirts of Comber.
Gritstone is used in the manufacture of asphalt, which is needed by Eurovia for the maintenance of roads in the south-east of England.
The managing director of Whitemountain Quarries, Mark Kelly, said the agreement will help to ensure the long-term sustainability of the Ballystockart site.
"It also demonstrates that our product has the seal of approval from one of Europe's leading road-building companies," he said.
"We see it as a positive sign for Northern Ireland's construction materials industry."
The Ballystockart aggregate will be shipped from Whitemountain Quarries' dedicated quay at Belfast Harbour to production plants in London, East Anglia and Humberside.
"We are delighted to extend our working relationship with Eurovia, who are involved in many major infrastructure projects in England," Mr Kelly said.
Whitemountain Quarries is the Quarry Materials division of the Lagan Group, one of Northern Ireland's leading construction companies.