Labour has urged the government to stop “ducking oversight” and “immediately publish” any evidence behind its controversial decision to exempt a new £800m defence research agency from Freedom of Information (FOI) law.

The move comes after ministers refused openDemocracy’s request to release the evidence behind the highly unusual decision to spare the Advanced Research and Invention Agency (ARIA) from being subject to transparency legislation.

All public bodies are ordinarily subject to FOI but ministers have previously insisted that ARIA, the brainchild of Dominic Cummings, should not have “the burden of processing” information requests.

But the Department for Business, Energy and Industrial Strategy (BEIS) has refused to confirm whether any evidence ever existed to back up the decision to exempt the “high-risk, high-reward” scientific research agency from FOI.

Labour’s Ed Miliband has called on the business secretary, Kwasi Kwarteng, to “immediately publish the evidence that informed his decision-making".

"The government cannot keep ducking oversight,” the shadow business secretary told openDemocracy.

Miliband pointed out that the US Defense Advanced Research Projects Agency – widely seen as the model for ARIA – is subject to American FOI legislation.

“There is no justification for a blanket exemption for ARIA from FOI laws, and this decision has no precedent,” he said.

Tom Brake, a former Liberal Democrat MP and head of Unlock Democracy, said that “the most likely explanation for BEIS's reluctance to make public the logic behind the proposed exemption of ARIA from FOI legislation is that there is none.”

The move comes amid a backdrop of growing concerns around government secrecy. openDemocracy recently won a major legal victory to force transparency on the Cabinet Office’s controversial FOI ‘Clearing House’ operation, which is due to be the subject of a parliamentary inquiry.

Brake said that the government is facing “inevitable” legal action over ARIA’s FOI exemption. Unlock Democracy and the Citizens, with digital rights agency AWO, sent a pre-action letter to ministers to force them to reveal how the new research agency is to spend its budget. The business department argued that secrecy was necessary to “empower exceptional people to focus solely on funding ground-breaking research”.

Under FOI legislation, openDemocracy had requested copies of evidence that informed the government’s decision to exempt ARIA from FOI but BEIS refused to provide any evidence, citing the need for “a safe space to debate live policy issues away from external interference”.

Government officials have said they “expect the number of FOI requests to be disproportionate to [ARIA’s] size and therefore inhibiting”. But similar UK academic research councils have, on average, received less than 50 requests a year, according to research by the Campaign for Freedom of Information.

The campaign’s deputy director, Katherine Gundersen, told openDemocracy that “blanket secrecy will only fuel suspicion that ARIA’s exemption from FOI is based on nothing more than ministerial prejudice.

“The government says ARIA is not being brought under FOIA to spare it the ‘burden’ of responding to FOI requests, but you can bet there's no evidence for that. Comparable bodies receive very low levels of requests.

“The great secret that is being concealed is probably that no one in the government believes a word of the government’s explanation.”

Labour’s John McDonnell, a member of the influential Public Administration and Constitutional Affairs Committee that is examining FOI, told openDemocracy the government “appears to be deciding ever more strained excuses for its refusal to uphold the basic principles of our freedom of information legislation.”

“This simply undermines still further confidence in the integrity of government decision making,” he added.

Legislation to set up ARIA is currently going through Parliament. Transparency campaigners have called on government and opposition MPs to block the move to exempt the agency from FOI.

Concerns over the state of FOI in the UK have been growing. An openDemocracy report, ‘Art of Darkness’, which was published last year, found that FOI response rates were at their lowest level since legislation was enacted a decade and a half ago.

A BEIS spokesperson said: “As the bill makes clear, ARIA will be subject to scrutiny by the National Audit Office, accountable to select committees and Parliament, required to submit an annual report and statement of accounts, and will have powers to introduce conflict of interest procedures to provide strong assurance of good governance.

“Together, these provisions are rigorous and proportionate, and will ensure that the research community, Parliament and taxpayers are informed of ARIA’s activities and where it spends its money.”