An ongoing row regarding an almost decade-old loan in Zambia has raised real issues over Europe's multilateral financial architecture.

The European Investment Bank, an EU member state-owned lender, has come under fire from a coalition of NGOs for failing to disclose the results of its own investigation into alleged tax evasion involving a mining project to which it lent $50m in February 2005.

The mine in question is owned by Mopani Copper Mines, which in itself is 73.1%-owned by Glencore, one of the world's largest commodity trading houses.

A leaked 2009 document from accountancy firm Grant Thornton suggested that Glencore had helped its subsidiary avoid tax payments totalling tens of millions of pounds and had "resisted the pilot audit at every stage". Swiss-based Glencore has denied the allegations.

The EIB launched its own investigation into the claims in 2011, but last week published a release stating: "Given that the EIB no longer has any contractual relation with Mopani/Glencore, and that these matters concern Mopani/Glencore's relations with the Zambian authorities, the bank has not taken any further view on this and considers this case as closed."

The logic seems to be that since Glencore/Mopani repaid the loan early (at their own request), the EIB has absolved itself of the task of ensuring that all tax was fully paid, despite the fact that it helped fund the project.

Despite this, the bank froze all new loans to Glencore and its subsidiaries in 2011, calling "serious concerns" over its corporate governance.

The masthead on the EIB's official website reads: "The EU bank" and campaigners are now calling on it to stop "disregarding its duty" of transparency, which should come with representing the half billion citizens of the EU.

However, a recent move by the EIB to revise its transparency policy will make the institution more opaque still, claim campaigners. The revision, they say, is a direct response to ensure the kind of leaks that plagued the Zambian project never happen again.

NGO Counter Balance, which lobbies the EIB to become more transparent in its dealings, claims that the changes will make the EIB "one of the most secretive financial institutions in the world".

"We think it's [the change in policy] related to the Mopani/Glencore deal," Berber Verpoest, a spokesperson for the Counter Balance told IBTimes UK. "The changes they're making now is to give more control of the bank as to what it makes public or not. We think it should be the other way around, we want an automatic mechanism that makes all information more public."

Verpoest said that even staff members of the EIB had pushed for fuller disclosure over the Zambian project, saying that the opacity compromised their work, but faced resistance from within.

The senior economic justice advisor at Christian Aid, Joseph Stead, added: "Depressingly, we have heard that this Zambian case is one of the reasons why the bank is proposing to become more secretive. We urge the bank's top managers to think again and to embrace greater transparency."

The Aid Transparency Index 2013 rated the EIB as having just 26.64% transparency, placing it sixth out of seven international financial institutions (behind the World Bank, IMF, IFC, etc). Some 45% of its data was not published in any form in 2012.

At a time in which the public anger over tax evasion is more visible than ever, this is clearly not good enough.

Earlier this year, a report by the Washington DC-based think-tank Global Financial Integrity found that African countries are losing up to 12.7% of their national GDP through illicit trading practices, a large chunk of it in missing tax revenues, facilitated by lack of transparency.

"The most basic rule of efficient markets is transparency. Opaque markets misallocate resources, result in inefficient decisions and can lead to corruption. That's what could well be going on here. While we continue to have substantial opacity in the world, these kinds of things will continue to happen and I have no doubt they are happening," tax campaigner at Tax Research LLP Richard Murphy told IBTimes UK at the time of its release.

The UK's Chancellor George Osborne sits on the board of governors for the EIB.

"I regard tax evasion and – indeed – aggressive tax avoidance – as morally repugnant," said Osborne in his 2012 Budget speech. If he's serious about this, then he and his European counterparts should be using their influence within the EIB to become transparency in all of its dealings: commercially sensitive or otherwise.

*Note the EIB was given the chance to respond to the content of this article but as it went to press, had failed to do so.