The competition watchdog has fined three firms more than £2.6 million for colluding to fix prices of liquid storage tanks.
The Competition and Markets Authority (CMA) said Franklin Hodge Industries, Galglass, KW Supplies and CST Industries conspired to divide customers among themselves, to fix the price of tanks and "to rig bids for contracts between 2005 and 2012".
Franklin Hodge was hit with a £2 million fine, Galglass must pay over half a million and KW Supplies £22,248.
CST Industries was granted immunity from fines under the watchdog's leniency policy for blowing the lid on the cartel and co-operating with the investigation.
Separately, Balmoral Tanks has been fined £130,000 for taking part in an "unlawful information exchange" regarding pricing and future pricing intentions with Franklin Hodge, Galglass, KW Supplies.
Stephen Blake, senior director of the CMA's cartels and criminal group, said: "Exchanging information with competitors, even at a single meeting, can infringe competition law with serious consequences for the businesses involved.
"Any company that is approached to join a cartel, or become involved in anti-competitive arrangements, should immediately reject the approach clearly and unequivocally.
"It should also decline to participate in any discussions that involve the sharing of confidential and competitively sensitive pricing information."