The London Metal Exchange has discovered bags of stones instead of the nickel that underpinned a small handful of its contracts at a warehouse in Rotterdam, in a revelation that will deliver another blow to confidence in the embattled exchange.

The amount of metal represents just 0.14% of live nickel inventories on the LME, however metals traders generally consider LME contracts to be beyond question, so the news that even a few of them could contain irregularities is hugely problematic. The news comes weeks after trader Trafigura Group revealed that it had been the victim of a vast alleged fraud involving missing nickel cargoes.

Contracts on the LME, which are the global benchmarks for industrial metals including aluminum, copper and nickel, are underpinned by physical metal in a network of exchange-registered warehouses. Any trader holding a contract to delivery receives a parcel of metal in an LME warehouse.

The issue affected nine contracts, representing 54 t of nickel, which is worth about $1.3-million at current prices. For the LME, though, the issue is a major headache, coming as it is still wading through the fallout of its last nickel crisis — the cancellation of billions of dollars in trades after prices spiked last year. Its regulator, the Financial Conduct Authority, earlier this month announced its first ever enforcement probe into an exchange over the LME’s conduct in the nickel crisis.

The issues were discovered at a facility in Rotterdam operated by Access World, according to people familiar with the matter. A spokesperson for Access World declined to comment. The company was previously owned by Glencore, and said in January it had been acquired by Global Capital Merchants.

The LME said there is no reason to believe that other warehouses are affected, but the exchange has asked that licensed operators recheck warranted nickel, it said in a statement. It noted that the issue related to nickel briquettes packed in bags, and so other metals, which do not allow bagged delivery, “are not susceptible to this type of irregularity.”

The LME warehousing system has over the years proven resilient to wider instances of fraud in the metals industry, which typically involve falsification of shipping and storage documents. The LME system is viewed as more secure because the exchange creates its own warehouse warrants and transfers ownership of them digitally, but it relies on warehousing companies to check the material as it enters their facilities to ensure the integrity of the market.

That includes weighing the bags that move in and out of the system. However, instead of nickel, the affected bags contained stones instead, according to the people familiar with the matter.

The irregularities “were evident from, among other things, the weight of the bags,” the LME said.

In its notice, the LME said the issues were discovered after it received information about problems with a number of nickel shipments out of a specific facility run by one of its licensed warehouse operators. The nine warrants have been invalidated and the warrant owner has been notified, the LME said.

The LME said it would delay by a week the long-awaited resumption of Asian hours trading for its beleaguered nickel contract, which had been due to restart on Monday.