Swiss commodity trader GlencoreXstrata owns 73.1% of Mopani Copper Mines, the mining firm investigating by the European Investment Bank for tax avoidance. Reuters

The European Investment Bank is facing allegations that it participated in a cover-up of facts in its report on allegations involving tax avoidance by a Zambian mining firm.

According to The Guardian, the EIB lent the Zambian firm Mopani Copper Mines $50 million (£30 million) to renovate a smelter in 2005. The updates were projected to reduce sulfur dioxide emissions from the mine, owned mainly by the Swiss commodity trader GlencoreXstrata. Six years after the work was completed, a leaked audit report hinted that Mopani failed to pay tens of millions of dollars in local taxes, leading the EIB to announce its investigation into the company and halt any loans to GlencoreXstrata, then known as Glencore.

Though the investigation was launched in 2011, the EIB has yet to reveal its findings on the allegations against Mopani. In response to the lack of information from EIB, 11 NGOs have written the bank’s president, Werner Hoyer, requesting the report’s release. In the letter, the NGOs reveal they have serious worries about “the secrecy surrounding the bank's investigation" and how the absence of any documents from the investigation undermines the EIB’s mission to offer "the highest possible levels of transparency in all its activities."

"It is now close to nine months since Christian Aid made a formal complaint to the bank about its failure to publish the Mopani-Glencore report. Despite having had this considerable period of time, the bank still has not replied to the complaint. We consider this an inexplicable and unacceptable delay," reads a portion of the letter. It ends with: "We cannot conceive of anything that would justify such secrecy and we therefore urge the bank to reveal the truth by publishing the report as a matter of urgency."

The document was signed by Alliance Sud, Les Amis de la Terre, Christian Aid, the Global Alliance for Tax Justice, Counter Balance, Oxfam, the Centre for Trade Policy & Development, Sherpa, Déclaration de Berne, Tax Justice Network Africa and Eurodad.

The Guardian reports the initial allegations stemmed from a pilot audit report that was leaked. The 2009 audit claimed that Mopani’s costs between 2006 and 2008 experienced an “unexplainable” increase, allowing the company to lower its claimed profits and taxes. According to Christian Aid, the EIB not publishing the report equaled a cover-up of the information that it was of aware of regarding the allegations against GlencoreXstrata.

An EIB spokesman admitted the bank did know about the NGO’s letter. He also said that the bank didn’t publish the outcome of its own investigation.

"A complaint process by the bank's independent complaints mechanism regarding the disclosure is currently in its final phase. A decision is expected in the following weeks,” the spokesman said. The EIB is also aware of the complaint being handled by the European ombudsman on this matter. The Bank will naturally cooperate with the European ombudsman in the handling of this complaint."

GlencoreXstrata has repeatedly denied the allegations and declined to comment on EIB’s actions. The company stressed that it fully cooperated with the 2011 investigation and has claimed the allegations were the result of a flawed “desktop study.”