Hedge fund giant Ken Griffin is leading the fight to stop a move by U.S. regulators that would force traders to pay into a new market surveillance system, according to a report.

Griffin’s Citadel Securities wants to block new rules that would require investors to put up money for the so-called Consolidated Audit Trail system, which is estimated to have already cost $1 billion, the Financial Times reported on Tuesday.

Known as CAT, the system provides regulators with a real-time view of all investor and transaction activity across all U.S. markets, including the New York Stock Exchange and the NASDAQ.

It has also raised privacy concerns, with Bill Barr, Donald Trump’s former attorney general, calling it an “unprecedented” step toward “an Orwellian surveillance state.”

Regulators have access to the identity of the traders involved.

They say they can use that data to tackle illegal practices such as stock manipulation and insider trading.

Citadel, whose founder Griffin has an estimated net worth of $43 billion according to Forbes, called the move “brazen” in a letter to regulators last week, the FT reported.

The letter said the rules, which were approved last September, amounted to “an attempt to extort hundreds of millions of dollars from brokerages like Citadel Securities.”

Exchanges currently pay the costs of the system, known as CAT, but the Securities and Exchange Commission, Wall Street’s top regulator, allows them to recoup two-thirds of those funds from traders.

The first invoices are expected to be sent in October and are based on this month’s trading volumes.

The Miami investment giant wants the SEC to suspend the plans until judges make a final ruling on their legality.

Griffin also joined the American Securities Association in October to bring the case, which calls CAT a “sham” and claims it is unconstitutional.

According to a court document, the Washington-based body failed to address “widespread investor concerns about transparency, governance, costs and data privacy.”

Exchange officials quoted by the Financial Times dismissed Griffin’s concerns.

“We’re just recouping our costs. There’s no profit here,” said a source directly involved in the project. “They’ve done every maneuver they can to avoid paying for the CAT.”

CAT provides regulators with real-time visibility into all investor and transaction activity across all U.S. markets, including the New York Stock Exchange and NASDAQ.

It also gives them access to the identities of the traders involved.

They say they can use that data to tackle illegal practices such as stock manipulation and insider trading.

The idea for CAT was first floated under the Obama administration in 2012, but it didn’t become fully operational until 2022.

It was seen as a response to the 2010 “flash crash,” which temporarily wiped nearly $1 trillion in market value from major U.S. stock exchanges.