The Competition and Markets Authority discovered that price fixing meetings went on for seven years from 2006 in the pre-cast concrete drainage sector.
Derbyshire-based Stanton Bonna Concrete Ltd and Somerset-based CPM Group Ltd have admitted breaking competition law by taking part in a cartel.
A third company is also under investigation and has not made any admissions.
The products were used in large infrastructure projects across Great Britain, including water management, roads and railways.
Typical customers include engineering and construction companies; utilities providers; and local and national government.
Michael Grenfell, Executive Director of Enforcement, said: “Cartels damage competition and lead to less choice, less innovation and increased prices for customers.
“We’ve provisionally found that these three firms secretly shared out the market and colluded on prices for construction products used in many building projects across Great Britain.
“The CMA does not tolerate such practices and will use our enforcement tools to crack down on those it believes are taking part in illegal cartels.”
Stanton Bonna and CPM have admitted to participating in the alleged cartel and have agreed to pay fines, which will be determined at the end of the CMA’s overall investigation.