Europol, the European Union’s law enforcement arm against organized crime, announced on Wednesday that carbon-trading fraud has cost the bloc’s governments $7.4 billion in lost tax revenue over the last 18 months.

“We have an ongoing investigation,’’ said Soren Pedersen, Europol’s chief spokesman, in a telephone interview on Thursday from The Hague. “We’re afraid the fraud is not completely finished yet, unfortunately. But it’s positive to see that actions are being taken and we hope soon it will disappear.”

Four Six member countries have changed their tax codes to protect against a recurrence, the agency said. The fraud involved adding the European Union’s value-added tax to the price of carbon dioxide permits sold to businesses. Fraudulent brokers then disappeared before turning the tax over to the government, according to Europol.

Many polluting businesses in Europe are required to buy the permits, which are part of a cap-and-trade system to reduce greenhouse gas emissions and combat global warming. A similar system is under consideration in the United States.

Europol, whose full name is European Law Enforcement Organization Cooperation , said that France, the Netherlands, Britain, and most recently, Spain have suspended or otherwise altered the application of value-added tax on carbon emissions permits. Belgium and Denmark are also changing their codes, Mr. Pederson said. The idea is to require the seller of the credits, rather than the buyer, to pay the tax. Such moves have resulted in a 90 percent decline in trading volume.

The agency’s announcement comes four months after British customs officials arrested seven people suspected of dodging taxes that should have been paid for selling the carbon permits.



Whether more such arrests were coming Mr. Pedersen would not say, but he suggested that the amount stated in that case did not represent the full amount Britain has lost to fraud. “That was some time ago,” he said.



Europol also said on Wednesday that it had since begun collecting and analyzing trading information with the cooperation of Belgium, Denmark, France, the Netherlands, Spain and Britain to help stop fraud. The agency warned that organized criminals might soon move toward the gas and electricity markets as the tax laws for carbon markets tighten .

“These criminal activities endanger the credibility of the European Union emission-trading system and lead to the loss of significant tax revenue for governments,’’ Rob Wainwright, director of Europol, said in a statement.

Europol said it noted suspicious trading in late 2008 when several trading markets had unprecedented rises in volume. That peaked in May of this year.

The European Union market, created in 2005, has been criticized for volatility and for rewarding some of the largest utilities with windfall profits. This year, some analysts said the market was finally showing signs of working, by pushing companies to switch to cleaner technologies, like using natural gas instead of coal, to produce electricity.

In Europe, there are six trading platforms, in Britain, Norway, Germany, Austria, the Netherlands and France. The European Union’s cap-and-trade carbon market, created to reduce climate-changing emissions, is worth about $132 billion a year, Europol says.