SINGAPORE (Reuters) - Commodity trading house Noble Group said it decided to take a $200 million impairment charge relating to an Australian coal investment only on the day of its earnings release, responding to a Business Times report on March 2, 2015, questioning the lack of a profit warning.

In a statement to the stock exchange late on Wednesday, Noble said it conducts an impairment review of long-term assets every quarter and had the option of impairment charges for its stake in Yancoal of between zero and $200 million, based on different assumptions.

The board and management decided on Feb 26, the day of its fourth-quarter and 2014 results, to adopt "the most prudent assumptions" and took an impairment charge of $200 million, Noble said in the statement.

In all, Noble took US$438 million (S$599 million) in asset writedowns for the fourth-quarter, which pushed it to a net loss of US$240 million, its first quarterly loss in three years.

The unexpected loss came after an obscure research firm, Iceberg Research, launched attacks on Noble's accounting practices, but Noble has said that its decisions on impairments were not related to the allegations.