Switzerland's competition authorities have broken up a cartel of Swiss and German road-surfacing companies, which had conspired to keep prices at 50 per cent above market levels.
The competition commission said the asphalt suppliers had made an illegal profit of SFr1.5 million ($880,000) a year since 1996.
The head of the commission, Roland von Bueren, said: "For the first time, we have caught out an entire industry in a large and important region."
However, he added that the commission was unable to fine the companies. Under Swiss law cartels can only be penalised once an injunction has been issued against them and in this case none had been put forward.
The commission said three Swiss companies had conspired to buy at least 30,000 tonnes of asphalt each year from two German suppliers, on condition that the German companies did not sell their products to any other buyers in Switzerland.
It said this arrangement had allowed the Swiss companies to mark up the asphalt to about SFr110 per tonne - roughly 50 per cent more than the German suppliers would have charged.
Von Bueren said he expected prices for road maintenance and building in northeastern Switzerland to fall now that the cartel has been broken up.
The Swiss firms involved are BHZ Baustoff Holding, BHT Baustoff Holding Thur and Biturit. The German suppliers are Asphaltmischwerke Bodensee and Friedrich Storz Strassen.
swissinfo with agencies