Eleven aid agencies and campaigning groups have urged the European Investment Bank to release the findings of its investigation into allegations of tax evasion by a Zambian mine majority owned by miner and trader GlencoreXstrata.
The EIB launched its investigation after a Zambian-commissioned audit of Mopani Copper Mines plc was leaked onto the web.
The audit was carried out by Grant Thornton and Econ Poyry, whose report said not only that "the existence of Mopani expenses is in doubt" but also questioned "the completeness" of Mopani's stated revenues.
Inflating expenses and artificially reducing revenues have the effect of fraudulently reducing a company's overall tax bill.
The leaked report also questioned the way in which the copper from the Mopani mine was sold to its parent company, Swiss-based Glencore, which later merged with Xstrata.
While subsidiaries are permitted to sell goods and services to their parent company, the price at which they do so must be arranged 'at arm's length'.
This is to guard against transfer mispricing, a phenomenon in which goods are transferred from a high tax jurisdiction to a low tax jurisdiction before being sold on the world market at high prices.
As a result of large-scale transfer mispricing, resource-rich developing countries are thought to be losing out on billions of dollars of tax revenue which could be spent on much needed public services.
"The Company (Mopani) has not been able to provide us with evidence that the Company sales, which mostly are related party transactions, have been entered into according to the 'arm's length' principle," the report said.
"ZRA (Zambia Revenue Authorities) should consider whether this should have impact on the tax assessment," the report added.
GlencoreXstrata categorically rejected the findings of the auditors' report, which it said were based on "fundamental factual errors".
The EIB is funded by members of the European Union and was created to promote EU interests by providing loans on favourable terms. The bank lent Mopani $50 million in 2005 to partly fund the renovation and modernisation of a smelter.
The EIB announced in May 2011 that it was investigating Mopani as a result of the auditors' report. It has completed its investigation but has not released the results.
Christian Aid, one of the signatories to Thursday's letter, first asked the bank to release the report in June 2013 but the bank refused to do so.
In their letter, the development agencies say that they, and the people of Europe, have a right to see the EIB report.
"The people of Zambia have a right to access information about the conduct in their country of a major multinational which is profiting from their natural resources," the letter says. "The people of Europe meanwhile also have a right to know what the truth is, given that their Governments are the Bank's shareholders."
Contacted for comment by Thomson Reuters Foundation, an EIB spokesman said the bank was aware of the letter but that "internal investigation reports have never been published by the EIB."
The campaigners' letter also said the EIB's refusal to publish the report was in breach of its own transparency policy.
"The policy states that the Bank is 'committed to achieving the highest possible level of transparency in all its activities' and that it recognises that 'openness on how it makes decisions, works and implements EU policies strengthens its credibility and accountability to citizens ... '" the letter said.
"The policy also states that 'all information held by the EIB is subject to disclosure on request, unless there is a compelling reason for non-disclosure'," the letter added.
Christian Aid has since complained to the European Ombudsman, an independent body that holds EU institutions to account, in an attempt to force the EIB to release the report. The body opened an investigation into the EIB's handling of the issue on March 14.
"The EIB is also aware of the complaint being handled by the European Ombudsman on this matter. The Bank will naturally cooperate with the EO in the handling of this complaint," the EIB spokesman said.
The letter was sent to EIB President Werner Hoyer, and was signed by the following development agencies: Christian Aid, Alliance Sud, Les Amis de la Terre, Global Alliance Tax Justice, Counter Balance, Oxfam International, Sherpa, The Berne Declaration, Eurodad, Tax Justice Network - Africa and the Centre for Trade Policy and Development.