The European Commission has fined five oil and chemical companies £519m ($682m) for price-fixing.
Eni, which faces the largest fine, says it might appeal
Italy's Eni faces the largest fine, of 272m euros (£183m; $357m), while Royal Dutch Shell was fined 160.8m euros.
The EU imposed the fine after learning that five firms were fixing the price of synthetic rubber used for tyres.
The fine is the second largest the commission has imposed for a cartel. It came after Germany's Bayer told EU lawmakers of the price-fixing.
The move gave Bayer immunity from any fine, which could have amounted to some 204m euros.
The fines for both Eni and Royal Dutch Shell were increased by 50% because they have previously committed similar offences.
The European Commission fined some of the region's top oil firms, including Shell and Total, for fixing the price of bitumen in September.
Eni, which said the fine was "entirely disproportionate and unjustified", has said it might look making an appeal against the decision.
Other firms fined over the latest price-fixing scheme include Dow Chemical, which has been ordered to pay 64.5m euros.
The figure was 40% below the original figure after Dow provided regulators with information in support of Bayer's findings.
Unipetrol, a Czech refining firm, has been told to pay 17.5m euros, while Polish firm Trade Stomil faces a 3.8m euro fine.
The largest fine imposed by the commission on a cartel came in 2001, after it ordered eight pharmaceutical firms to pay 790m euros for fixing the price of vitamins.