Tata Steel is refusing to comment on claims it has made £700m windfall profits from a policy designed to protect the climate.
Three separate experts say Tata made the cash by selling carbon emissions permits it was given for free.
They say Tata was allocated more carbon allowances under the EU emissions trading scheme (EUETS) than it needed.
There is no suggestion Tata broke the rules, and the firm said its permits were a "matter of public record".
Reports say Tata profited more than any other firm in the UK from the much-criticised trading scheme, which allowed it to sell the surplus to other firms wanting permits to pollute.
Other windfalls allegedly went to Lafarge, Hanson, and Total UK.
But the analysis is controversial because some climate sceptics blamed EU climate policies for the demise of Tata.
One report - by consultants CE Delft for citizens' group Carbon Market Watch - shows that the ailing steel firm was the UK's biggest beneficiary of the EUETS windfall, making £704m.
Overall it says industry across Europe has earned a £19bn windfall from 2008 to 2014.
Tata declined to comment on the specific figures, but said it disagreed with the methodology used in the reports for calculating its profits from the EUETS.
It said the allegations were a "selective telling of the many flaws in the EUETS."
A spokesman told BBC News: "We agree that the current emissions trading system needs to be fixed and all parties should focus on how best this can be done, rather than conducting theoretical studies on what it could have previously meant for individual companies."
The problem happened because when the EUETS began national governments were left to decide how many carbon permits their industries needed to cushion themselves from foreign competition.
The governments typically allocated more permits than necessary because it cost them nothing directly, whilst boosting domestic firms.
The sectors profiting most from pollution payouts have been iron and steel, cement, refineries and petrochemicals.
One report says the amount netted by industry over the period is more than 10 times the amount the EU has spent on innovation under the EUETS to make firms more energy efficient.
I understand Tata is telling critics privately that it cannot be blamed for legitimately exploiting loopholes in a badly designed system.
Green groups say this is disingenuous as major European polluters lobbied governments to give them more emissions permits than they would really need. (Smaller firms not facing competitive global pressure had to buy all their emissions permits).
'Bill' for taxpayers
The EUETS has long been criticized for its failure to reduce carbon emissions, and citizens' groups are pressing for it to be reformed again.
Femke de Jong, of Carbon Market Watch, said: "Instead of making the polluter pay, energy-intensive companies are allowed to pollute for free under the EUETS.
"Even worse, they are able to profit from their pollution to the tune of billions. It's European taxpayers that are picking up this bill as governments forego scarce public money."
Emil Dimantchev, carbon analyst from Thomson Reuters, estimates that Tata Steel is likely to have netted around £794m (€1bn) between 2008 and 2014 in windfall profits.
Dr Simon Evans, of Carbon Brief, calculated that across Europe Tata Steel made £1.4 billion from carbon allowances. His analysis criticises recent reporting of the steel crisis.
Tata Steel said: "It is a matter of public record that on occasions we have been allocated more permits than we used.
"There are a number of reasons why in some years we haven't used all of our permits, which includes increasingly efficient production techniques and the impact of the global economic and steel industry crisis.
"The latter means that our volumes of production, and therefore emissions, have been well below what anyone forecast and in the main they continue to be so.
"However it is important to note that the situation changes on an annual basis, so in 2014 Port Talbot was under-allocated and this meant that we had to spend scarce resources on additional permits."
Business Secretary Sajid Javid has said Tata Steel will begin the formal process to sell its loss-making UK plants by Monday.
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