By Anshuman Daga and Clare Baldwin

Noble Group Ltd on Monday asked a court to ban an ex-analyst and a firm he is associated with from publishing what it said was false information, after a slew of reports by an obscure research firm shaved off as much as a third of the commodity trader's market value over the past month.

The injunction, filed at a Hong Kong court, comes after Iceberg Research issued its third report about one of Asia's biggest commodity traders, saying Noble had substantially understated its gross and net debt.

Hong Kong-headquartered Noble has repeatedly rejected Iceberg's claims, but its market value has shrunk by as much as $1.8 billion since the mysterious research group accused it in mid-February of inflating asset values by billions of dollars through aggressive accounting.

Pressure also mounted on Noble after it unexpectedly reported its first quarterly loss in three years, and the slump in its share price has raised the prospect of a possible takeover, bankers told Reuters.

The stock was trading 5 percent higher on Monday.

In the legal writ, Noble named the defendants as Arnaud Vagner, a Hong Kong-based credit analyst it said it had hired in 2011 and then fired two years later, and a Seychelles-registered firm that Noble had previously said was behind Iceberg Research.

Reuters was unable to locate Vagner, or find details about him or the Seychelles-listed Enlighten Ace Ltd, despite repeated attempts.

Noble said the defendants had conspired to damage its business operations, financial position and drive down its share price by "anonymously spreading false and misleading information". Noble did not provide further details.

Iceberg also declined to give details about its members or organisation, but said its research reports were based on public information released by Noble.

"We think what matters is our arguments rather than the identity of Iceberg's members," it said in an email to Reuters. The company said it is not a short-seller in Noble's shares.

Iceberg's reports have raised investor concern about how much leeway commodity traders have in valuing their assets, with some brokers downgrading their ratings on Noble's stock.

Noble is the second major Singapore-listed commodity trader to have its accounting practices challenged by a research firm. Olam International Ltd was criticised by Muddy Waters in late 2012, sending its shares falling and prompting state investor Temasek to take majority control.