Elliott Associates, a U.S. hedge fund is suing the London Metal Exchange (LME) for $456 million for cancelling nickel trades following a spike in prices linked to Russia’s invasion of Ukraine.

The case claims the LME acted “unreasonably and irrationally” when it cancelled trades originally made on March 8, a move that the exchange said was meant to avoid disorderly trading.

The legal action piles more pressure on the exchange, which is being probed by regulators and is struggling to restore trust and volumes in its nickel market.

Elliott said the LME should not have halted trading and erased deals after prices more than doubled to over $100,000 a tonne in a matter of hours on March 8.

The LME and LME Clear Limited were named as defendants in the judicial review claim filed in a British court by Elliott Associates and Elliott International last week, the LME’s parent company Hong Kong Exchanges and Clearing said.

The Guardian reported that the lawsuit will pile further pressure on the LME, which is also facing a UK regulatory investigation over the suspension and cancellation of those trades.

The LME suspended trading in nickel for a week in early March after prices doubled over the course of a day. The surge in prices was caused by uncertainty over the metal’s availability, given fears that companies such as the Moscow-headquartered Nornickel – which is world’s largest supplier of the metal – could face sanctions as a result of Russia’s invasion of Ukraine.

The exchange reopened trading on March 16, but caused further uproar when it cancelled all transactions that were made on the first day of the suspension on March 8. The LME has defended its decision, saying it needed to protect the market and prevent smaller members of the exchange from going bust.

“This decision to suspend trading was taken because the nickel market had become disorderly,” the LME’s parent company, Hong Kong Exchanges and Clearing (HKEX), said in a statement.

“Cancellations were made retrospectively to take the market back to the last point in time at which the LME could be confident that the market was operating in an orderly manner. It should be stressed that the LME always acted in the interests of the market as a whole.”

Elliott, which is the world’s largest activist hedge fund and is led by the billionaire Republican party donor Paul Singer, has alleged that the decision may have violated Elliott’s “human rights,” according to the HKEX announcement regarding the lawsuit.

A spokesperson for Elliott said the hedge fund considered that the LME “acted unlawfully in that it exceeded its powers when it cancelled those trades, or that it exercised the powers that it did have unreasonably and irrationally, in particular by taking into account irrelevant factors (including its own financial position) and failing to take into account relevant factors”.

The LME said it believed Elliott’s claim was “without merit” and said the exchange would “contest it vigorously”.