The Competition and Markets Authority handed down an £8m fine to H.J. Enthoven – trading as BLM British Lead – and a £1.5m fine to fellow Hertfordshire supplier Associated Lead Mills (ALM).
Both firms admitted their roles in the illegal cartel earlier this year, reducing their potential fines.
Previously, investigators at the Government’s cartel watchdog, provisionally found that a third supplier, Calder Industrial Materials, had become involved at a later stage in one of the arrangements.
But the investigators have now determined that there are no grounds for action and have closed the case.
Four anticompetitive arrangements took place between October 2015 and April 2017.
These included colluding on prices, carving up the rolled lead market by arranging not to target certain customers, and arranging not to supply a new business because it risked disrupting the firms’ existing customer relationships.
Each of the arrangements saw commercially sensitive information exchanged.
Michael Grenfell, executive director of Enforcement at the CMA, said: “These companies knowingly entered into illegal arrangements restricting competition between them.
“Such anti-competitive arrangements tend to inflate prices and cheat customers out of a fair deal. The CMA does not tolerate such behaviour.
“Construction is a sector firmly under our spotlight and if businesses break the law by entering into anti-competitive arrangements, they run the risk of large fines.”