An 18-month battle to discover the true cost to consumers of building the Hinkley Point C nuclear reactors is to come to a climax in London.

The Information Commissioner has been blocking freedom of information requests to publish subsidy documents held by the Department of Energy and Climate Change.

However, it has finally agreed to an oral hearing on the issue before the Information Tribunal.

The decision comes just before EDF’s chief executive in Britain, Vincent de Rivaz, faces a grilling from MPs about the £18bn cost of Hinkley, which is underwritten through subsidies and customer bills.

EDF is yet to make a final decision on whether to go ahead with the project and MPs have said there are “serious questions” to answer about Hinkley’s viability.

Thomas Piquemal, EDF’s finance director quit two weeks ago over concerns about Hinkley’s impact on the already heavily indebted company. However, David Cameron and François Hollande have strongly backed the project as “a pillar of the bilateral relationship” and “a key aspect of Britain’s energy policy”.

The government has agreed to pay £92.50 a megawatt hour for Hinkley’s electricity in 2012 pounds, index-linked to the Consumer Price Index. That was about double the wholesale price when the deal was reached but now represents almost three times the wholesale power price due to rises in the CPI and power price declines.

Today’s school leavers will be paying for HPC until they draw their pensions

Greenpeace and Request Initiative, a Freedom of Information Act specialist, has been trying since 2014 to obtain the contents of seven documents that are understood to contain further details about the subsidies for Hinkley.

They were submitted to the European Commission as justification for the need to provide state aid, which is generally against competition rules.

Greenpeace said it was extraordinary that the Information Commissioner had been supporting DECC’s wish to keep vital information away from the public. The environmental group now hopes progress can be made at an Information Tribunal hearing expected to take place in London in May.

Doug Parr, policy director at Greenpeace, said: “Bizarrely, the Information Commissioner and DECC are hell bent on keeping the evidence showing Hinkley is a good idea for Britain a secret. The reports we have been trying to see for 18 months illustrate the assumptions that DECC used to decide that Hinkley is the best bet to power Britain in the future.

“We think it’s hard to think of any reason it should be kept secret unless the evidence shows that Hinkley isn’t such a good idea. The argument it has to remain a secret because it would damage the government’s negotiations is now over because the negotiations have finished.

“Today’s school leavers will be paying for Hinkley until they’re about to draw their pensions. The government should tell future generations what they are paying for and why.”

Commissioner: ‘commercial secrecy trumps public interest’

The information in the documents was largely prepared for ministers by advisers such as KPMG, Poyry and Oxera, with modelling work and scenario planning by DECC itself.

The preliminary view of the bureaucrats in Brussels was that the measures constituted unlawful financial support, but the final decision reversed this and gave the UK government the green light.

Greenpeace asked for access to the documents and then launched a Freedom of Information request for them to be handed over, which was opposed by DECC.

The Information Commissioner initially agreed to withhold the documents on the grounds that “the reports contain and discuss matters of a sensitive commercial nature and the information was provided to DECC in the expectation that it would be treated as confidential”.

It also accepted that the disclosure of the information would “disadvantage the government in the context of future negotiations with other (nuclear plant) developers”.

The Commissioner said there is a strong public interest in disclosing the contents of the documents. but said this was overridden by the other considerations around commercial sensitivity.

But despite this, it has accepted that an oral hearing is “appropriate” in this case. The Information Commissioner said he had no further comments to make ahead of the hearing but DECC justified its commitment to keeping the documents under wraps.

DECC: ‘commercial sensitivity’

A spokesperson for the department said: “The decision to withhold these documents is due to commercial sensitivity in accordance with exceptions set out in the Environmental Information Regulations.

“The Information Commissioner supported this decision and we appreciate that this is now is the subject of a live appeal and we can’t comment any further whilst the appeal process is ongoing.”

EDF itself has always argued that it is totally committed to transparency, saying this would separate the nuclear industry from the secrecy that characterised it in the past.

Last September de Rivaz said: “At EDF Energy, transparency is at the heart of everything we do. It is our responsibility, as the custodians of Britain’s nuclear fleet, to be transparent, open and receptive to questions.

“We are approaching the Final Investment Decision for our new nuclear project Hinkley Point C. As we do, scrutiny has naturally increased. Just as we embrace transparency, we welcome scrutiny. We relish challenge based on facts.”

With two legal challenges agains the Hinkley C subsidy package proceeding, the importance of the so far secret documents cannot be overstated.

If it turns out that they contain significant errors of fact or argument, based on which the Commission decided that the HPC subsidy package was legal, then this would assist the litigants greatly in making their claim that the Commission’s approval was legally invalid.

Terry Macalister is Energy Editor at the Guardian. He tweets @TerryMac999.

This article was originally published on the Guardian and is republished here with thanks via the Guardian Environment Network. This version includes some additional reporting by The Ecologist.